A lot of stuff found its way on the DS and other systems after a fashion
Tetnus on drugs/Lockjaw-
http://filetrip.net/gba-downloads/homebrew/download-lockjaw-the-overdose-milestone-4-f5072.html
Vulkanon and bulletGBA- two fine bullet hell shooters.
http://pqrs.org/gba/vulkanon/index.html.en http://pqrs.org/gba/bulletgba/index.html.en
Herg's solitaire
http://www.oocities.org/hergtoler/gbdev.html
I liked elite but that would be an example of another system (though you might have to work a bit harder to find it), if you can think of a classic game from the Amiga and similar vintage PC through 16bit/PCE era there is a good chance a version of it appeared on the GBA as homebrew as well (
http://www.portabledev.com/pages/gba/jeuxdev.-perso/chip-advance.php being one of my favourites though you might prefer the DS version called codename hacker- same author), many of these were stellar.
Trade winds (though some prefer the commercial game Sea Trader Rise of Taipan) was very nice.
Emulators were another thing I found enjoyable but those are not really worth much I figure, some of the demos however are well worth checking out.