http://www.capstone-engine.org/index.html

Capstone is a lightweight multi-platform, multi-architecture disassembly framework.

This isn't a particular tool but it's what you would use to create other tools and it's got some smart folks involved with it.

Support hardware architectures: ARM, ARM64 (aka ARMv8), Mips & X86 (more details).

Clean/simple/lightweight/intuitive architecture-neutral API.

Provide details on disassembled instruction (called “decomposer” by others).

Provide some semantics of the disassembled instruction, such as list of implicit registers read & written.

Implemented in pure C language, with bindings for Python, Ruby, OCaml, C#, Java and GO available.

Native support for Windows & *nix (including MacOSX, Linux, *BSD platforms).

Thread-safe by design.

Distributed under the open source BSD license.