CC65 is a C cross-compiler for 6502 systems.  CC65 includes a
powerful macro assembler, a debugger, an archiver, and a highly
configurable linker.  The linker supports overlays, ROMable code,
split output, among other features.  The compiler supports a large
subset of ISO C (as per the ISO 9899 specification).

It is easy to write code for standalone 6502 boards (target none).  In
addition, particular support for the following targets is included:

        - C64
        - C128
        - C16, C116, Plus/4
        - CBM 600/700 family (programs run in bank 1)
        - Newer PET machines (not 2001)
        - The Apple ][

The assembler and linker also support 65816 as a target.

WWW: http://www.von-bassewitz.de/uz/cc65/