Atari 2600

Acorn Virtual 2600 - http://www.mnementh.co.uk/v2600/
Emulator port for Acorn RISC OS.

Atari 2600 Emulator for GBA - http://www.otakunozoku.com/Articles/ToyBox/Atari2600VCSEmulatorForGBA/Atari2600VCSEmulatorforGB.html
Emulation project for Game Boy Advance with details and screenshots.

Atari VCS Preservation Retrogaming Archive - http://atarigamer.tripod.com/
514 ROM Images in one zip file. [Requires Flash]

AtariAge: 2600 Emulation - http://www.atariage.com/2600/emulation/
Resources and links to the most popular emulators and utilities.

Dan B's Atari 2600 Tech Page - http://atarihq.com/danb/a2600.shtml
An open source emulator (Virtual VCS) for DOS and technical information.

GP32 Atari 2600 Emulator - http://www.atariage.com/2600/emulation/GP32/
Port of Virtual VCS emulator to the GP32.

Imdylanks Atari Roms - http://www.geocities.com/imdylank/user.html
A collection of ROM Images packed in three files.

Paul Robson's A26 - http://members.aol.com/autismuk/emu.html
DOS based open source emulator.

Stella - http://stella.sourceforge.net/
An open source emulator for DOS, GP2X, Linux, MacOS, Unix, Windows, and Windows CE.

StellaX - http://www.emuunlim.com/stellax/
Windows 9x, NT and 2000 port of Stella.

Virtual 2600 - http://www.hornby.dsl.pipex.com/v2600/
Historical look at Alex Hornby's Virtual 2600 emulator. Virtual 2600 is an open source emulator for DOS and Unix.

z26 - http://www.whimsey.com/z26/
An open source emulator for DOS, Linux, and Windows based on Paul Robson's A26.