The GP32 is a small scale console released for the homebrew market, to moderate success. While commercial games are available for it, its strength lies in emulation; practically any 8-bit platform you can think of has emulation software compatible with it, include NES, SNES, Genesis/Megadrive, ZX Spectrum, Commodore 64, and many others.
The console looks remarkably like a Gameboy advance, proving that design follows function, but the screen is much larger and backlit, making things much easier to see. The GP32 runs on AA batteries, four of which last about 5 hours, so keep a good supply to hand, or buy an AC adapter. Games are easily transferred over via PC.
Some of the emulation software isn't all it claims to be; for instance, the SNES emulator is rather buggy and flawed, but others such as the NES work fine, and the Genesis emulator is perhaps the smoothest I've seen.