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.
Easy for use.