xa (Andre Fachat's open-source 6502 cross assembler)

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.

This build includes xa's companion piece, the disassembler dxa. dxa is
considered alpha-quality software, but is included because it's still
very useful. It produces output that can be reassembled with xa, but
doesn't support the 65816 CPU.