cc65

cc65 is a complete cross development package for 65(C)02 systems, including a powerful macro assembler, a C compiler, linker, librarian and several other tools.

cc65 has C and runtime library support for many of the old 6502 machines, including the following Commodore machines:

• VIC20
• C16/C116 and Plus/4
• C64
• C128
• CBM 510 (aka P500)
• the 600/700 family
• newer PET machines (not 2001).
• the Apple ][+ and successors.
• the Atari 8-bit machines.
• the Atari 2600 console.
• the Atari 5200 console.
• GEOS for the C64, C128 and Apple //e.
• the Bit Corporation Gamate console.
• the NEC PC-Engine (aka TurboGrafx-16) console.
• the Nintendo Entertainment System (NES) console.
• the Watara Supervision console.
• the VTech Creativision console.
• the Oric Atmos.
• the Oric Telestrat.
• the Lynx console.
• the Ohio Scientific Challenger 1P.

The libraries are fairly portable, so creating a version for other 6502s shouldn't be too much work.