xa is a high-speed, two-pass portable cross-assembler.  It understands
mnemonics and generates code for:

  NMOS 6502s (such as 6502A, 6504, 6507, 6510, 7501, 8500, 8501, 8502, ...)
  CMOS 6502s (65C02 and Rockwell R65C02) and the 65816

Key amongst its features:

  - C-like preprocessor (understands cpp for additional feature support)
  - Rich expression syntax and pseudo-op vocabulary
  - Multiple character sets
  - Binary linking
  - Supports o65 relocatable objects with a full linker and relocation
    suite, as well as "bare" plain binary object files
  - Block structure for label scoping

WWW: http://www.floodgap.com/retrotech/xa/