New F12 Developer Tools
If you're a developer, be sure to check out IE11's new F12 developer tool suite, which before this has remained unchanged since IE8. Microsoft also has a website (www.modern.ie) that helps with crossbrowser checking, as well as addressing of common problems that arise from supporting older versions of IE. As the time of writing, the website has been updated with a new screenshot service and new virtual machine images.
IE11 includes a completely re-designed set of F12 developer tools for Web developers to diagnose and optimize their apps.