Asm-Pro is a modern, maintained, feature-rich asmone derivative.

Another modern/maintained one, focusing on low end 68000/68010 machines, is asmtwo.

AsmOne was a very popular assembler at the time, and has seen a few derivatives. I remember an old one called trash'm'one.

Good luck! As the sibling comment mentions, Asm-Pro is IMO a good choice these though it requires KSv2+ (so you can't use it on an stock A500). I mostly use vasm for "larger" stuff though (http://sun.hasenbraten.de/vasm/) and cross-compile (though it can run on Amiga as well) / test in an emulator.

For a quick test/code loop nothing beats the Asm-* family - remember to save often and make backups though :)