ATARI 2600 / 5800 / 7200

You can find a DOS Atari emulator here. It is called Z26 and also works in Windows.
Since I do not host illegal material, you will need to find the games on the internet or make your own backups.
These emulators and source data files are here for informational purposes only.
Stella is a multi-platform Atari Emulator that is the emulator of choice for me.

For information on all things atari, visit Atari Age
Atarimania is a comprehensive database/collection of all things Atari-Related including Roms, Snapshots, Instructions, User Guides, and more.
There is also a Wikipedia entry on Stella the emulator itself!