Homebrew, most stop at LSDJ (
http://www.littlesounddj.com/lsd/ ). There are a few other pieces of homebrew, mainly on the slideshow, basic implementation of a classic game (there was a good slide puzzle one), demoscene and people fiddling with dev kits/assembly. You might also have to look at "PD"/public domain as a search term as homebrew was not really a term in the GB/GBC lifetime.
Still
http://pdroms.de/category/gameboy gives a nice overview and
http://www.zophar.net/pdroms/gameboy.html is worth a look as well.
ROM hacks/translations.
If it is not on
http://www.romhacking.net/ and you are not looking for pokemon hacks then it probably does not exist (though their pokemon hack collection does week out the "changed the starter and nothing else" nonsense quite well), if you are looking for pokemon hacks pokemon hacking sites are the better bet. Pokemon hacking is fairly advanced for GB/GBC entries so there is quite a bit.
My favourite ROM hack still has to be the "Link gets laid" hack, however you probably want to have played the main game first.
Afraid I am not so familiar with the leaked, beta and unlicensed dumps stuff for the GB/GBC as I am for the GBA and DS. However if I do not mention Tyrannosaurs Tex (
http://gbatemp.net/threads/gbc-first-person-shooter-tyrannosaurus-tex-finally-gets-released.344635/ ) then I will probably be asked to leave the site, you might also like the Resident Evil game if you otherwise like the series (though you will want
http://www.romhacking.net/hacks/891/ as it was unfinished), it will certainly be more enjoyable than the actually released title that goes by the name of Resident Evil Gaiden...