Simd is free open source library in C++. It includes high performance image processing algorithms. The algorithms are optimized with using of different SIMD CPU extensions. In particular the library supports following CPU extensions: SSE2, SSSE3, SSE4.1, SSE4.2 and AVX2 for x86/x64, VMX(Altivec) and VSX(Power7) for PowerPC.
Nice library, but lacking examples & docs. However it is quite easy to use.