BarsWF is the worlds fastest md5 bruteforcing password cracker, just in case you didn't already know.
It combines using your computers processor with you graphics cards GPU for computing the largest amount of hashes as quickly as possible. It is not unheard of to get 100's of millions of hashes per second when using this application.

Prerequisites which really dont need to be mentioned are a processor with SSE2 instruction set, and any nvidia geforce 8 and up graphics card with CUDA support if you want the really fast one, otherwise most decently new nvidia graphics cards bought in the past 2 years should be able to work with this.