So you want to compete in SCD's challenges? You've come to the right place, then!
This is the GBA emulator section, where you can download software for emulating the Game Boy Advance handheld console.

Downloads/Emulators/Game Boy Advance/

Visual Boy Advance 1.7.2
Game Boy Advance emulator. Doesn't support Netplay or input recording, unfortunately, but it'll do everything else.

Visual Boy Advance-M
GBA emulator with input recording support. This will, hopefully, be the emulator SCD will use for its GBA Sonic challenges in the future.

Visual Boy Advance Link
This is an experimental extension to Visual Boy Advance that has support for linking VBA emulators. SCD may be able to use this in the future to host online GBA tournaments. We at SCD have not yet decided on this yet, though. Until then, you can use the above two emulators.

*Note: There are currently no challenges for Game Boy Advance Sonic games, however, there will be in the future. ;)