1. Clean Code (assistant)
   If you've read Robert "Uncle Bob" Martin's Clean Code book, then you'll find this extension extremely useful. If you have a method that is way too long, it underlines the method. If you have a number of methods chained together, it'll notify you. Overall, this is a great extension to keep your code in check and your refactoring always churning.
   
   
2. ForTea
   When I use T4 templates, it can be messy if your color-coding isn't enabled to show you your mistakes. ForTea includes the ability to edit .tt, .ttinclude, and .t4 files, syntax highlighting for C# and T4 directives and blocks, and Intellisense completion. Using ForTea has saved my sanity a number of times while working on code generation templates.
   
   
3. ReSharper Powertoys: Cyclomatic Complexity
   While we all try to make our code easier to use and maintain, the only way to gauge the complexity of your code is to perform code reviews. 
   
   
4. ReSharper: Refactor JS
   The Refactor JS extension allows users to refactor JavaScript converting property access from reference to indexed form and vice-versa. It also removes unreachable code.
   
   
5. ReSharper: Utility Pack
   This extension pack adds additional context menu refactorings including: Duplicate Method, Invert Return Value, Make Abstract, Make Virtual, Pull Parameters, Reverse For-Loop, Use As Operation, Use Cast Operation, Use String.Compare, and Use StringBuilder.

Bonus Extension:

• Clippy for ReSharper!!
  HOW (not) AWESOME IS THIS?!?! Yes, they have Clippy helping you with coding. Scary I know, but the extension was specifically built as a tutorial to show you how to create a JetBrains ReSharper Extension.(Source code: https://github.com/citizenmatt/resharper-clippy)
  
  
  

If you want to create your own ReSharper plugin, their SDK is available here.

Did I miss any or do you think there is a better one out there that you use? Post it in the comments below.