Selenium Webdriver is an automated testing platform for web pages. It may not be the easiest or the most user-friendly, and it definitely requires a lot of coding, but it's the most powerful and works with any website. Even better, it "drives" a real web browser and simulates real clicks, so you're getting a real UI test, not just a URL test.
It's good stuff, but it's also arcane. That's why I'm putting together this handy cheat sheet. Expect it to change over time.