Virtual GameBoy Advance is a good GBA emulator, available for several OS's. Unfortunately, the Windows port is shareware and costs $35, making it the only commercial GBA emulator out there. The Linux and FreeBSD ports are actually up to par with the windows version and are freeware. There's also a freeware DOS version, but it has been discontinued after v0.6. Nevertheless, the freeware ports are really worth checking out (try the DOS version with a frontend, for example).
