diStorm is a lightweight, easy-to-use and fast decomposer library.
diStorm disassembles instructions in 16, 32 and 64 bit modes. Supported instruction sets: FPU, MMX, SSE, SSE2, SSE3, SSSE3, SSE4, 3DNow! (w/ extensions), new x86-64 instruction sets, VMX, AMD's SVM and AVX!
diStorm is written in C, but for rapid use, diStorm also has wrappers in Python/Ruby/Java and can easily be used in C as well. It is also the fastest disassembler library!
Note that diStorm3 is backward compatible with the interface of diStorm64 (however, make sure you use the newest header files).