So not only is the 2600 vigourously sought out in flea markets and traded and discussed on the web, many people still try to program for this creature. Why? Nostalgia? Fun? I really don't know. And I'm programming a game! ;)
Most people who program these games use a PC. There are many very nice programs for Atari 2600 game developement on the PC. The most popular ones are PCAE (PC Atari Emulator by John Dullea) and DASM (an assembler written by Matt Dillon for the Amiga and ported to PC by Bob Colbert). If you have a PC, get off this page!!! ;) Go to Bob Colbert's page for these tools or Nick Bensema's SUPER PAGE!!! for your one-stop PC, C=64, Amiga, and Linux 2600 developement page!
The problem was that there wasn't anything out there for us people crazy enough to own a Mac and want to program the Atari. And so my quest began. (yes, that was said very tongue in cheek)
After a nearly exhausting search of the net ;), I finally found everything you need to program and playtest 2600 games on the PowerMac. No such luck for the 68k, however. There just isn't an emulator for it, and now things are looking slim that there'll ever be. But hey, PowerMac's are below $500 now (for old, discontinued models) so break out your ImageWriters and old 640x480 67 Hz-only monitors and hook up to a new Mac!
Anyway, MacStella 0.7 and RASM via MPW are the tools of choice for the Mac Atari programmer now. MacStella makes your computer behave as if it were an actual Atari 2600... with a twenty year old processor and television interface coming through your monitor. RASM does everything DASM did, but runs in the MacOS, and everyone knows that alone will save you headaches. MPW is large, but is incredibly flexible. After you get used to assembling games for the Atari, it's not too big a step to start compiling C and Pascal programs for your Mac itself! So jump in, read the page, download the goodies, and hop on it! That game you always wished would come out on the Atari is about to!