AS6502--A Cross-Assembler for the 6502
Supports undocumented opcodes, includes source and executables for 16-bit DOS.
Bob Colbert's Web Page
Bob Colbert's Stella Resources, including commented source code and the DASM Assembler.
How I Built My Custom Atari 2600 Cartridge
Includes images, diagrams, and links.
MAS65--6502 Macro Cross Assembler
For Z80 CP/M and DOS systems, source available.
Nick Bensema's Atari 2600 Programming Page
Tools, sample sources, disassembled games, and home-brewed binaries.
Stella: The Starpath CD Mailing List
Support group for programmers of Atari 2600 games.