Atari Linux Emulators

I am helping this guy transfer over to Ubuntu. One of his big questions is if there are Atari Emulators? His dad wrote Atari games and he would like to see if he could port the games over to the linux machine. So I guess my question is, ... "Is there a good Atari emulator and how could he port the games over to Ubuntu?"

Thanks!
Keeping the evil penguins at bay,
PenguinHunter

See if this helps. I am at work so the links are blocked.
http://ubuntuforums.org/showthread.php?t=327495