JetBrains ReSharper which really enhances the IDE and helps with best practices, refactoring, etc. (It looks like DevExpress CodeRush is another alternative)
RedGate .Net Reflector to help understand the ins and outs of how things are working "behind the scenes".
GhostDoc is nice if you're using XML comments to document your code.
TestDriven.NET is great if you're doing Test Driven Development (or any kind of Unit Testing, really).
And while I haven't used it personally (but plan to try it, now that I found it), MetalScroll looks like a very helpful tool when working with large code files. It replaces the scrollbar with a scroll-able document preview.