Liboil is a library of simple functions that are optimized for various CPUs. These functions are generally loops implementing simple algorithms, such as converting an array of N integers to floating-poing numbers or multiplying and summing an array of N numbers. Clearly such functions are candidates for significant optimization using various techniques, especially by using extended instructions provided by modern CPUs (Altivec, MMX, SSE, etc.).
The library is available as 32-bit and 64-bit: