Clover is a code coverage analysis tool. It discovers sections of code that are not being adequately exercised by your unit tests. Clover reports its findings in multiple formats for easy use by the whole team at the project level down to each line of source code.
This started out as an internal tool developed by these guys for their own use and over time decided to turn it into a product. They tell me they have been pretty successful todate. Good one guys!
And the really good news, they are busy working on a Clover for .NET.