Metasm: https://github.com/jjyg/metasm/
Radare: http://radare.org/
Capstone: http://www.capstone-engine.org/
Capstone is based on LLVM, that you cannot beat in term of architectures and industrial quality, and has great plugins, which make it kind of my favorite.
Of course, for non-scripting needs, you need decent graphical interfaces, as provided by these ones:
IDA: https://www.hex-rays.com/products/ida/
Hopper: http://www.hopperapp.com/
IDA is clearly the best, but Hopper is a fair choice if you need it and you can't afford IDA license for personal use.
[Edit: add forgotten IDA and Hopper...]