**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.