Pef for fun - an interesting tool to promote TDD
the PEX team has released on online version of their unit testing tool at http://www.pexforfun.com/
I found this to be a real fun way to explore and understand how Test Driven Development is all about.
having said that, i'd like to add that PEX is much more than the above site. for more details on PEX, you can go to their main site @ http://research.microsoft.com/en-us/projects/pex/