Is there an assembler for which a
majority of available code was written for? Probably the most famous
bit of 6502 asm out there is the Apple II Prince of Persia
The mnemonics for the 6502 instruction set were pretty well known. As far as macros and
such, back in the early 80s I wrote a 6502 assembler in C that ran on our 11/34 (unix),
and used the M4 preprocessor. This was in support of software development on Commodore 64
and superpet systems.