Two really cool features in Visual Studio 2010 for C# developers. 

1) Sticky Notes Quick Watch with annotations.  You may say, huh?  What are you talking about?  Well, look at the screen cap below.

This is by far one of the coolest features I have seen so far (apart from the awesome XAML / WPF designer).  When debugging you can save a quick watch as a sticky note in your code (the note disappears when you exit debug mode).  You can even add notes to the watches so that next time you debug you can know what the values are supposed to be.  Instead of leaving bookmarks all over your code you can add a sticky note! 

2) Highlight to debug.  You can now highlight a section of code (including casts) to view the properties and values.  This was available in VB for some time but finally is working correctly in C#.