Portable Atari 800 emulator. Pocket Atari is based on this emulator. From the homepage you can visit developers mailing list and check CVS source repository for the latest updates.
Atari800Win Original Win32 port of Atari800 by Rich Lawrence. PLus version made this one obsolete but you still should pay a visit to Rich. Don't forget to check WinFrotz -- the best Infocom adventure game interpreter for Windows.
Atari800Win PLus If you are Windows user you definitely should give this one a try. It is based on the same core as Pocket Atari, slightly different version. Currently it is the most advanced Atari800 port with the easiest to use user interface.
Emulators, Inc. Developer of PC XFormer -- one of older Atari 800 emulators for DOS. Later this emulator was ported for Windows and Win32. It lacks some features of Atari800Win but it is definitely not bad. It is also faster than Atari800, so if you have slower system this one may be preferred choice.
Atari800 for Agenda Here's another Atari800 port to PDA-type device. This time it is Linux-based Agenda VR3. Also, Jiri Svoboda is working on port for Linux-running iPAQs, no link yet.
Retrogames Emulation news website. If you are really interested in emulation you should come here often. You will find a lot of useful information in forums. There is even PocketPC section on the site.