Unit testing is obviously something all of us developers should all be doing... and if you aren't, maybe a good book will help you start.
Roy Osherove is writing a book on Unit Testing (writing, as in he hasn't finished yet ) But, you can get the first chapter for free, and the rest of the chapters as they are written via the MEAP (Manning Early Access Program).
Part I Getting started
1. The basics of unit testing - FREE
2. The first unit test - AVAILABLE
Part II Core techniques
3. Using Stubs to break dependencies - AVAILABLE
4. Interaction testing Using Mock Objects - AVAILABLE
5. Mock Object frameworks - AVAILABLE