CRASM is a portable cross assembler for 8-bit CPU(s): 6800/6801/6803/6502/65C02/Z80, written by Leon Bottou. This is a forked version of the original version 1.5 sources and compiled for 32 bit Windows using MinGW.
You can find the original source code and distribution here: http://crasm.sourceforge.net/crasm.html
The Windows executable available here was compiled using MinGW.
CRASM's output is in Motorola S-Record format or in Intel's HEX format. If you'd like to convert the S-Record format to straight object/binary format, check out Ant Goffart's SREC2BIN utility, available here: http://www.s-record.com/
Using CRASM and SREC2BIN can give you a convenient way to compile assembly for older computers such as the Tandy Radio Shack TRS-80 Micro Color Computer MC-10 (uses the Motorola 6803), TRS-80 Models I, II, III, IV (uses the Z80), and the Apple II (and other 6502 based computers, Atari 400/800, etc.).