Vector Class Library, latest version
This is a C class library for using the Single Instruction Multiple Data (SIMD) instructions to improve performance on modern microprocessors with the x86 or x86/64 instruction set on Windows, Linux, and Mac platforms. There are no plans to support ARM or other instruction sets.
Add-on packages for particular applications
Getting-started video. Video blogger Christopher Rose has made this nice video telling how to get started with the Vector Class Library.
Help: You may ask for programming help on StackOverflow using the tag vector-class-library.