Sauce Labs - Test mobile or web apps instantly across 700+ browser/OS/device platform combinations - without infrastructure setup.
BrowserStack - Test your website for cross browser compatibility on real browsers. Instant access to multiple desktop and mobile browsers. Get Free Trial.
Selenium - Selenium automates browsers. That's it! What you do with that power is entirely up to you. Primarily, it is for automating web applications for testing purposes, but is certainly not limited to just that.
browserling - Live interactive cross-browser testing from your browser.
TestComplete - TestComplete Desktop, Web, and Mobile helps you create repeatable and accurate automated tests across multiple devices, platforms, and environments easily and quickly.
CrossBrowserTesting - Browser Testing made simple! Run automated, visual, and manual tests on 1500+ real browsers and mobile devices. Test more browsers, in less time.