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.

  - GEOS for the C64 and C128.

  - the Nintendo Entertainment System (NES).

  - 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.