I will leave others to suggest homebrew although I will suggest you visit:
http://www.playeradvance.org/
If you mean GBA homebrew (in which case might I suggest you leave out supercards) go with
GBA slot carts: pre DLDI homebrew. Prior to addition of DLDI to libfat and its predecessor GBA_NDS_FAT or a library based upon it were used for homebrew. The best bet for these homebrews is a CF using GBAMP or maybe a m3-cf or supercard CF as some of the later cards did not have libraries (or support from libat) at the time. Arguably the only thing of merit from back then (i.e. something without DLDI support) is a rom dumper or rein/rain (the save tool, EZ4 version (the only one I have at present) is here:
http://nds.cmamod.com/2006/11/24/ez4-can-w...ven-faster-now/ )
RAM, various homebrew makes use of the RAM present on various GBA slot devices to do various things. Usage is low and methods fragmented. Lick's RAM api seeks to reduce that:
http://forum.gbadev.org/viewtopic.php?t=13023
Personally GBA slot is just as viable as DS slot in my opinion (for everything) and has the added bonus of GBA rom and homebrew support assuming you get the right cart (various GBA lacking versions exist).
As for actual carts various threads always seem to boil down to following
m3 perfect or EZ4 (original, lite or lite deluxe as lite compact is the reduced GBA version): if you have the cash and like external memory: do it (EZ4 deluxe is the "best" of the EZ4 line).
g6: same as above but with the limitation of internal memory (the 512 megs you have are there always). Cost may be above the 2 choices above (including a decent memory card) and once again old homebrew may well be out (the m3/g6 people were very slow getting a decent library/source code out for the g6).
EZFlash 3 in 1: lacks support of old DS homebrew and has limited memory for GBA (you have to store it on your DS cart) but does do GBA and RAM functions and is damn cheap. Needs a DS to be written to as well.
People tend to dismiss supercards due to their issues with RAM (it is there and OK for homebrew but GBA is done far better by others).