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The gp2x is a Linux based open source handheld console.
It plays pretty much everything from XviD/DivX videos to MP3 and OGG audio files. Plus there are hundreds of free games available, emulators of old machines like Sega Genesis, SNES , Amiga, C64 , Atari ST, ... you can even play your beloved oldschool adventure games like Day of the Tentacle on this lovely machine.
Other superior features include the usage of standard rechargeable AA batteries and standard SD cards as memory, making the whole machine quite a cheap alternative compared to other proprietary standards used in other handhelds.
More info online in the Wikipedia article about the gp2x.