reSharper (R#) is the best Visual Studio productivity plugin hands down. It greatly simplifies such typical programming tasks as code refactoring (moving code around, renaming classes, members, variables), contextual search (syntaxis-aware code search). reSharper can also analyze code on the fly and offer optimizations and improvements.
A lot of solutions, first offered with reSharper, eventually migrate to Visual Studio itself. As such it can be said that reSharper developers are one of the main innovators in the field of .Net programming tooling.