The Atari 800 was announced in December 1978, though didn't actually start shipping until late in 1979. It was an interesting machine in that it used multiple special-purpopse coprocessors for sound and graphics to take the load off of the 6502 CPU. This allowed the 800 to do quite impressive animation and graphics for it's time. I remember typing in a BASIC program from one of Compute's! 'Books of Atari' series that, using the 800's display list, is able to display 128 colors at one time, in the shape of the Atari FUJI! The 800 also had provisions for sprites and collision-detection built into it's hardware, so it was an excellent machine for game programmers. It's version of 'Frogger' is still considered by many to be the definitive version. Like the Atari 400, the 800 was designed by a team which included Jay Miner, who later went on to help design the Amiga.
There were a lot of interesting add-on hardware available for the 800 as well, from the IndusGT disk drives, which had their own Z80 cpu, to digitizing tablets such as the Koala Pad, which included it's own paint program on cartridge, to enhancements to existing hardware, such as the 'Happy' chip for the Atari 810 disk drive, which allowed copying of almost any Atari 8bit disk. The 800 was also the only machine in the Atari 8bit series to offer 2 cartridge ports, as well as the only one to use plug-in cards for it's ROM and RAM expansion. Most cartridges used only the left cartridge slot, but there were a few that used the right slot as well, such as 'Monkey Wrench' from Eastern House, and 'Magic Dump' from Geminisoft. These programs could only be used on the 800 since no other Atari 8-bit had the dual slots.
Some of the capabilities of the machine include (as taken from the Atari 8-bit FAQ):
Atari dropped all remaining support for the 8-bit computer line on January 1, 1992. Atari was bought out by disk drive manufacturer JTS Corp. on July 30, 1996, and production of it's computers stopped. The Falcon was sold to C-Labs of Germany who enhanced it and continued it's production. On February 23, 1998 JTS sold it's Atari division to Hasbro Inc. for $5 million, forming Atari Interactive Inc. Atari Games, the coin-op division which remained seperate from Atari Corp. and was later known as Time-Warner Interactive, became a subsidiary of Midway Games Inc.