ReSharper

I'm a huge fan of ReSharper from JetBrains. I've been using it for over three years now and I think I'd be lost without it. Take away my ReSharper and I might as well have notepad.

I think my top five most used features are:

1. Ctrl+left mouse click. This takes you to the thing you clicked on's definition.
2. Ctrl+E+C. Code cleanup. I have a custom rule that is called 'Everything but 'var'.
3. The NUnit integration. If you use NUnit in Visual Studio, you need ReSharper.
4. Find Usages
5. Ctrl+R+R. For renaming.

This is by no means an exhaustive list. It's just probably what I use most day in, day out. Just go buy it. I promise that the first time you use a foreach snippet after defining something that's IEnumerable you will be smitten.