Atari has recently decided to release 100 of their all-time hits into the Android Market. These seem to be predominately the Atari-made arcade and 2600 versions of games, so certain titles didn't make the cut.
In any case, it's a sign of the times that my HTC EVO 3D packs enough processing power to run 296 Atari 2600s simultaneously on the dual cores (not even considering GPU) and has 8,388,608 times the RAM.
Atari's Greatest Hits is probably best for nostalgia purposes and not gaming, as there's a lot missing without the cheap plastic enclosed in thin rubber controller. These games also translate oddly to the small screen, though with most things, your experience may vary wildly.
When you download the free app, you get Missile Command for free too. Any additional games can be purchased in packs of 5 for $1, or you can get all 99 for $9.99.
Should you be wondering, a complete list of games can be found in the Market and yes, Yar's Revenge is on there.