Extensions for Visual Studio

• PowerCommands - Useful extensions for Visual Studio (Supports VS 2012) adding additional functionality to various areas of the IDE
• DevExpress CodeRush Xpress
• AnkhSVN - Subversion Support for Visual Studio
• Ghost Doc - Simplify your XML Comments
• Visual Studio Color Theme Editor - Make your Visual Studio look pretty with themes
• VsVim
• DPack
• VSFileNav - fast searcher with wildcards + camel case searches
• Sonic file finder - fast and convenient search
• AllMargins
• tangible T4 Editor plus modeling tools for VS2010 adds IntelliSense and Syntax Coloring to T4 Text Templates
• Word Wrap with Auto-Indent
• Indentation Matcher Extension
• Structure Adornment
  • BlockTagger
  • BlockTaggerImpl
  • SettingsStore
  • SettingsStoreImpl
• Source Outliner
• Triple Click
• ItalicComments
• Go To Definition
• Spell Checker
• Remove and Sort Using
• Format Document
• Open Folder in Windows Explorer
• Find Results Highlighter
• Regular Expressions Margin
• VSCommands
• HelpViewerKeywordIndex
• StyleCop
• Extension Analyzer
• CodeCompare
• Team Founder Server Power Tools
• VS10x Selection Popup
• Color Picker Completion
• Numbered Bookmarks
• Mouse Zoom
• Visual Studio 2010 Pro Power Tools
• JSEnhancements - provides outlining and matching braces highlighting features for Visual Studio JavaScript editor; provides fantastic #region collapsing in JS and CSS files, making long files much easier to handle;
• Code Contracts Editor Extensions
• WoVS Quick Add Reference
• JScript Editor Extensions
• Align By
• T4 Editor
• Quick Open File for Visual Studio 2010 - quick opening any solution file
• CleanProject - Cleans Visual Studio Solutions
• NuGet - NuGet is a free, open source developer focused package management system for the .NET platform intent on simplifying the process of incorporating third party libraries into a .NET application during development.

Not Free

• Resharper
• Visual Assist X
• JustCode
• ViEmu
• CodeRush with Refactor! Pro
• VisualSVN
• VS10x Code Map - Displays a graphical nested representation of the current editor window code (Supports Visual Studio 2012)
• VS10x Editor View Enhancer (Supports Visual Studio 2012)

Get more details on MSDN: Installing and Managing Visual Studio Tools and Extensions