So lets say you have an idle GPU (graphics processor, your video card for example). And you want to try your hand at cracking some password hashes, where there is an opensource tool for your called hashcat. In particular OCL-Hashcat-Plus. Its built just for this purpose.
Why do I bring this up? Well there are allot of low-power, multi-core hardware projects coming out, and this could be a good use for them. Check back in 6 months for more.
http://hashcat.net/oclhashcat-plus/