Perry Marshall's Split Tester
Split Test 2 Ads To See Which One Will
Have A Higher Click-Thru-Rate Over Time

SplitTester.com - This is an awesome tool created by Perry Marshall and Brian Teasley. It's used when you're split testing two Google ads against each other. Using statistical significance the tool determines if one ad will have a higher click-through-rate over the other in the long run - or if it just got lucky by getting a bunch of clicks for a short period of time.