I second Fantasm, specifically 6.x. Get it from the Macintosh Garden and have fun!
As previously stated, MPW will do the trick, too.
There are more tools, but you'd have to do more digging... But I don't expect them to be better than the two above. (Did CodeWarrior also offer an assembler?)
Side-note: Technically we call "assembly compilers" just "assemblers". There's really no compilation happening (unlike in a processor-agnostic programming language), it's all 1:1 mapping between mnemonics ("text" instructions) and binary CPU OpCodes (binary instructions). ("Disassembly" being the term for the same 1:1 translation, but in the opposite direction.)