The listing, Mattel Juicebox has ended.
Directly from Wikipedia...
The Juice Box is a low cost multimedia player made by toy manufacturer Mattel. The player features a 2.7 in (6.9 cm) screen with a native resolution of 240×160 px and runs uClinux, a microcontroller version of the Linux kernel.
It has 66MHz ARM7TDMI architecture Samsung processor, 2 MBytes or 8 MBytes of RAM and 8MB of ROM. It was marketed as a portable media player for kids. The player only played a proprietary cartridge format. Nickelodeon, 4Kids Entertainment, and Cartoon Network put some of their shows on cartridges. ...
Original retail price was about US$70. As of 2008, the player is cheap enough to be a good platform for hobbyists to experiment with design concepts, as it uses a Linux based operating system.
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You can find some good links on hardware hacking this platform at elinux.org/JuiceBox or just google for it.
The included "cartridge" is actually just a blank plastic case that Mattel included to make you feel like you were getting a cartridge with the system.
Free shipping within the US only.