Capstone is a disassembly framework based upon LLVM that is multi-platform and multi-architecture. Capstone's goal is advertised as "[making it] the ultimate disassembly engine for binary analysis and reversing in the security community."
Capstone is implemented in pure C with bindings for various languages from Python to Java to Go to C#, has native support for Windows and Linux among other platforms, is thread-safe, and is BSD licensed.
This LLVM disassembly framework is almost up to version 2.0 per the 2.0 RC1 announcement from a few days ago. Highlights over the original Capstone include a 50% smaller library size, much less memory usage, performance improvements to the framework, PowerPC architecture support, new instruction support for ARM/ARM64/MIPS, critical bug-fixes, and architecture flexibility improvements.