AS6502--A Cross-Assembler for the 6502 - Supports undocumented opcodes, includes source and executables for 16-bit DOS. - http://www.whimsey.com/as6502/
MAS65--6502 Macro Cross Assembler - For Z80 CP/M and DOS systems, source available. - http://www.hytherion.com/beattidp/comput/x65tools/
Bob Colbert's Web Page - Bob Colbert's Stella Resources, including commented source code and the DASM Assembler. - http://members.cox.net/rcolbert/