"The CSS Editor for Visual Studio 11 Developer Preview is a complete rewrite of the 2010 version, featuring excellent performance and stability. As soon as you start to edit a CSS file or a style block embedded in a web page, you'll feel the difference!"
A playground for your html, css and javascript code. You can use this to create a small experimental code to test across all browsers. You can also share you code with others and work off others code (forking)
Creating a fancy text editor box using the Ajax control toolkit. I'm guessing you guys have your own favorites for this, but here's another option that's baked into VS (kind of)
Very useful jQuery simulator which lets you use different frameworks and many other options. Especially useful for sharepoint development, because deploying/closing sharepoint is slow in comparison.