acme (6502 family cross assembler)

ACME is a free cross assembler released under the GNU GPL. It can
produce code for the following processors: 6502, 6510 (including
illegal opcodes), 65c02 and 65816. ACME supports the standard
assembler stuff like global/local/anonymous labels, offset assembly,
conditional assembly and looping assembly. It can include other source
files as well as binaries while assembling. Calculations can be done
in integer or float mode. Oh, and it is fast.

Note: If you're looking for a Plan9-flavored text editor called acme,
you're in the wrong place. Search for acme2k. If you're looking for an
ACME protocol client for use with LetsEncrypt, search for acme.sh.