I found a great web app for testing regexes as-you-type, but have lost it in the months since I last use it. Searching through here, I've found http://www.rubular.com/ which is useful, but not what I found originally. Today, I found a few testing tools through google, but most often they are slow, don't allow you to test as-you-type, or just don't work.
What free, online, test-as-you-type regex testing tools do you use?