I'm a big fan of Cucumber, a testing tool in Ruby. Cucumber lets you write tests in a language that business people can read (Gherkin). This really helps when working out requirements and edge cases.
These days I am working .Net for my day job. So I was happy to find SpecFlow - Pragmatic BDD for .Net. SpecFlow implements Cucumber for .Net. I've tried it a couple of times and found it works really well. One more tool for my tool box!