browserling - Live interactive cross-browser testing from your browser.
Microlink screenshot - Take a screenshot of any website
Browsershots - Browsershots makes screenshots of your web design in different browsers.
Thum.io - A real-time website screenshot generator
BrowserStack - BrowserStack is a software testing platform for developers to comprehensively test websites and mobile applications for quality.
Screenlapse - Automatically capture website screenshots every day/week/etc