While doing research for a TDD presentation I'm giving this week, I stumbled across Bill Wake's site XP123. He has tons of useful tidbits in there, but the one that really caught my eye was his Test-First Challenge.

Basically it is a series of practice exercises for doing TDD. He provides the tests and you do the implementation. Check it out. (The tests are all Java, but it should be pretty straightforward to convert them to C#.)