There were a few different flash cart type forums over the years, some catered to slightly different things (more homebrew, more emulators, more Scene, more reviews, more game dumping) and had slightly different focuses in terms of what they looked at in terms of the consoles they covered. Likewise rivalry was never really a thing -- "build it and they will come" is not quite a thing on the internet but "build it, gather some followers and make sure you always have new content and they will come back" is one, it is for that reason we do not have a stupid rule like "do not link rival sites". Of course GBAtemp is hardly beloved by all with similar interests, varying reasons for this though I will mostly leave that to others for speculation.
All those alternatives have since died down, merged into GBAtemp after a fashion, become entirely different sites, started being basically archive forums and other such things. The closest you really get now is however you want to look at the likes of the 360 and PS3 modding sites and they inhabit a different realm, I am not sure what goes with some of the Wii stuff as the Wii never really had any interesting hacks once USB came along but in my "once every months" forays into various parts of the internet say not a lot. The obvious exception is for non English forums, and though GBAtemp has/had ties to the would be "rivals" as it happened these GBAtemp more often worked really closely with (not least of all because many of our staff do not have English as a first language). This is especially true in the case of the French (playeradvance I believe had Costello as staff for a time, though it is probably more familiar to those doing 360 related things logic-sunrise did not go unnoticed), Italian (though our founders were also Italian the folks at GBArl.it worked quite closely with us), German and Dutch/Belgian side of things. Sadly I have forgotten the names of most of the nice sites for the latter three (there is a nice German hacking site at romresources.net) but chances are if one of our members has one of the flags, a join date to match and a reasonable amount of posts they probably were involved. I never saw much from Spain (though elotrolado.net, being where Waninkoko released a lot of work, probably warrants a mention) or Portugal, anywhere else in the Med, The Nordic countries or that much of Poland. I can not say we ever did that much with Russia but many of the folks that frequented gbx.ru would drop in from time to time. In the end though most of the discussion happened in English and/or Chinese the GBA and co were so very far from a native English speakers only game I can not quite understate it.
With the exception of China (obvious reasons, though actually even there ROMs or no ROMs does not matter so much. TGbus, yyjoy, ndsbbs and others that still have ROMs on so no links/mentions) and maybe one other (ish, though most of that was "can't get it to run on my flash cart" discussions) we never saw a ROM site have a forum rise up to rival anything here. I am not sure why this is and it does not tend to happen elsewhere -- I can not think of a not categorically opposed to warez video or music forum that does not tolerate some links save for one pre database type site, though I must admit I am not really into that sort of thing. Also though I frequently pull nice tools/info from Brazilian hacking sites (read I mainly just scan through romhacking.trd.br) I have yet to find a rough equivalent to GBAtemp here.
To this day I have never found a Japanese ROM hacking, console hacking, homebrew liking or rough equivalent to any other site on this list that stuck around for long enough to get established. Plenty of single author stuff and the occasional forum that dealt with something kind of related (I have fished a fair few modding tools, specs and pieces of homebrew out of them over the years) but never something I could link to others or keep an eye on to make sure I did not miss out on something.
(more) names and some waffle then, there might be some order to parts of this but do not read anything into the orders within the groupings
High Society. They technically did release a few things but they also had some pretty good hackers among their forum goers.
Garpland. People have mentioned DS-scene as the GBAtemp banned members destination, some of the more infamous banned members ended up here some years before (can not remember if v2 or V3 for this one). At one time had something of a rivalry with no-intro (garp = gameboy advance redump project) but that was more philosophical than anything else.
TehSkeen. I never quite got a handle on Brakken and was not in the same IRC circles as him so what I have is largely third hand but he ran this site, it was more general and gamecube focused but nobody could deny a reasonable focus for the GBA and DS at the time.
DS-database. Probably the closest we ever got to real drama, those that know what went know what went and I am not interested in bringing it back up.
Pocketheaven. Though today most will probably know them more for their GBA and DS emulation forums they had one of the finest release lists, great patch collections, notable features like "how much Japanese you need to play this game" and other such things. At various points there were interesting ties to GBAdev which is but one of the things that informs my "in private, what various anti piracy hackers do is quite interesting" type thing I mention at points.
Eurasia. They are around, they are staffed by good people and though they are not that active I can not ignore them as part of this little exercise.
scorpei.com. The author floats around here from time to time and he had some nice stuff.
qj.net. I am never quite sure about what this site is/was. Generally it is a downloads/news site with a serious focus on homebrew, most people went there for PSP stuff but it by no means did badly otherwise.
maxconsole. Today something of a more legit board, frontend to/sideproject of a shop (though some would argue it always was) and general advert site it more or less held its own at points in time.
DC-emu. One of the few other sites to have ever gotten away with naming themselves after a console. Though most people do not recognise it as a rival per se I can safely say Wraggster knew his stuff, ran a tight news section and fostered a lot of projects.
The forums of various cheat making companies or ones tied to them are worth a mention. Then and now they have some good people doing good things. To this day I believe Parasyte is the only one to have ever got their DS banned from online activities (he was among the first to make cheats for the DS and they banned his mac, though as it was changed in seconds thanks to being in the firmware that did not matter). kodewerx was one of them, bookmarks that have long since been lost and that I do not tend to move in cheating circles have sapped my memory as far as others go though.
You should also note that GBA emulation, mainly via VBA, was great at the time (and basically available from day 0) and used by an awful lot of people -- flash carts were quite expensive after all and the GBA was a nice aspect ratio screen with a few buttons and no touchscreen that lent itself very well to emulation on a PC. As such the VBA forums were a place to be. ngemu.com and emutalk did not do too badly either for this sort of thing.
emucr.com probably warrants a mention.
http://jul.rustedlogic.net/ . A ROM hacking site of some note; it fractured, imploded and otherwise morphed several times over the years (
http://acmlm.kafuka.org/board/ probably being the most notable variation on the theme) but still exists. Likewise before romhacking.net and GBAtemp's own hacking forums rose up (and whirlpool.. who knows) there was some work done at zophar.net.
I have several more sites I do not quite want to see forgotten but many of those are ROM listers (advance-power.de, ADVANsCEne, ludibria.com) without terribly extensive forums, more general info sites (
http://www.gameboy-advance.net/ then and now is a great little resource), more homebrew focused sites of single people or small groups (
http://www.devrs.com/ ) or general homebrew sites (dev-fr, gbadev of course, pd-roms, drunkencoders.com and nintendomax.com probably being the big ones).
http://arcvoreppin.free.fr/ips/ I have to link just because I do.
http://reinerziegler.de/GBA/gba.htm ,
http://mootan.hg.to/fmgba/ and
http://hp.vector.co.jp/authors/VA018359/ are linked for similar reasons.
Today several of the games that get hacked extensively (mainly pokemon, fire emblem, Mario platform, mario kart, Golden Sun, megaman and Advance Wars) happen somewhat outside GBAtemp's normal sphere (I certainly rarely see others link to the sites on here, I often find good info otherwise unknown/unshared on GBAtemp on them and it seems to go the other way as well), I wander through them every few months to see if I can dredge up a new tool, technique or some such from them however.
Whatever remained of the PS1 era stuff largely seemed to die out, become an archive site of sorts (sadly my favourite, mainly as it hosts nice old tools and techniques, I probably should not link here) or maybe ended up on avforums.com. This just lands us right back in general hacking circles and I am sure we all know of the likes of se7ensins, xbox-scene, xbox-hq and xboxhacker.net (though there are a lot more 360 hacking sites than many realise and seemingly those on those others do not know of ones I would have considered key sites in the 360 scene) and have equivalents for the PSP, PS3 and such. Curiously though I have never seen a site that is the rough mirror of GBAtemp but for other consoles, most of it is a slightly different set of rules but ones that serve to fundamentally alter how things play out (some discuss online modding extensively, others ban all discussion of releases, some have a more cheat focused nature, some have a more modding tutorial focus and it goes on).
To the best of my knowledge aside from flash cart launches on various manufacturer sites being DDoSed there have never been true attempts at black hat nonsense (either technical or
http://cryptome.org/2012/07/gent-forum-spies.htm ) in these circles. Several have been hacked generally, several have had staff go rogue, what happens on IRC stays on IRC and a few have had legal and monetary issues of various forms but for the most part it has been fairly calm.
Thus we have the slightly sanitised version of the GBA, DS and (some) modding circles as understood and misremembered by FAST6191. As this has but a tiny mention of IRC and even less of the Scene it is probably useless as a form of history. I have a feeling I have given undue prominence to several sites and maybe missed out several of the bigger ones I really should have linked up but hey.