UncleBob's Three Rules of TDD
This was one of the three TDD cards I posted while at Object Mentor. A more attractive set of hand-drawn cards then appeared (with full attribution) at Brian DiCroce's site a little while after. I still wish I'd drawn them first. The other have been presented here: FIRST principles and our site's most popular card to date, the Red, Green, Refactor card.
These three laws originated with Robert "Uncle Bob" Martin, who has provided such a wonderful write-up that there is no value I can add other than shrinking the sentences to fit on a card. Bob is a great guy and a solid techie. And by solid, I mean SOLID.
By the way, I've submitted a paper to Agile2009 to do a little session on doing "in a flash" teaching sessions with index cards. If it's accepted, I'll see you there.