Visual Boy Advance is a sweet little emulator.
One of these days, I'm going to break down and buy a <a href="http://www.gbax.com/">Flash cartridge</a> for my wife's GBA, but not so I can put multiple GBA games on one cartridge...
...rather, so that I can put hundreds of <a href="http://www.pocketnes.org/">NES games</a> on one cartridge.
Then again, it's hard to justify spending the money for a GBA flash cartridge, when for not that much more I could get my hands on a <a href="http://www.gbax.com/gp32review.html">GamePark GP32</a> and play NES, Atari 2600, ColecoVision, Game Boy, Game Boy Color, TurboGrafx/PC-Engine, Sega Master System, Game Gear, Super Nintendo, Atari ST, Spectrum, MSX, LucasArts SCUMMV, some arcade games, some Id Software ports, DivX movies, AVIs, MPEGs, MP3s, and assorted other open-source projects on it.