Primecoin instead of being a standard hashcash type of coin has its users calculating prime number chains to find blocks.
As of the typing of this post, primecoin has risen all the way up to .015 BTC per XPM and fallen to its current level of about .01 BTC per XPM.
Primecoin is traded on most alt exchanges like coins-e and cryptsy. You can learn more about it at Reddit Primecoin and download the latest and fastest wallet at this Sourceforge.net for linux and This one for windows
