gp2x - Atari 400 / 800 XL XE emulation via atari800gp2x v1.0
Atari 400/800 and all of the XL and XE variants are emulated with atari800-gp2x. It'll even run the Atari 5200.
What's cool about emulating these 8-bit computers is that while only some games made it to the 2600, 5200, Colecovision, Intellivision, etc consoles, many of the games are also on the Commodore 64 and Apple II.
Although IMO, the C64 was a much better game machine. Check out this C64 video:
• gp2x - Commodore 64 emulation via frodo2x