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 5200 console.
- GEOS for the C64, C128 and Apple //e.
- the Nintendo Entertainment System (NES) console.
- the Supervision console.
- the Oric Atmos.
- the Lynx console.

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