I recently downloaded and installed the trial version of JetBrains ReSharper and I am so happy with it I can't see straight. This is by far the best developer plugin for Visual Studio that I have ever come across. I haven't paid for it yet ($149 for the C# edition, $50 more for the full version that covers VB as well), but I plan to.

I have a license for CodeRush, but I found the learning curve to be too extreme for me to live with it. I know it has a strong following and it seems like a powerful tool, I just didn't have a week to learn it. I also wasn't keen on disabling all of the things that annoy me about it, becuase I move from computer to computer often and don't want a laundry list of mods I have to make to the IDE before I can use it.

ReSharper just installs and works without completely changing the way I work. I have been using it for about two weeks and these are what I think are the most important features...

• Robust, Awesome intellisense inside ASP.net markup:
  You heard me! The script blocks inside AS*X pages is more than fully supported. Coding inside an as*x page is now just as nice as coding inside a .cs page. 
• Real time code analysis:
  As you type, ReSharper checks your code for errors and gives you immediate feedback in two places; Inside the text editor (squiggly line style) and in the Resharper gutter. No need to compile a huge project looking for compiler errors anymore. WHEE!
• Code completion on Steroids!
  I am leery of code completion without my explicit command, but I have gotten used to the way that ReSharper does it and I love it.
• Code Formatting:
  ReSharper gives you warnings (orange markers) for all redundant usages of "this." (they are everywhere if you are working on a team. People really seem to love "this."), unused uses clauses, redundant delegate assignments, etc... Heck! It even analyzes your xml files and tells you if they are well formed and if not, where any issues may be. Brilliant!

There are many more features listed in their feature list, but those are the ones that have most impressed me so far. Download it and give it a try. It'll make you a better, more efficient developer. GET IT!