Check out this post by Carl J with links to 15 free Visual Studio add ins. Some of the ones I am interested in, and have just installed:
Code Style Enforcer: very cool! make your code more friendly!
GhostDoc: auto generation of XML Documentation.
Refactor! for ASP.NET: this very rocks!
Regionerate: tool to quickly apply layout rules to your code. handy.
Resource Refactortoring tool: Working in Localization, as i do, this is handy. allows you to remove all hard coded strings and place them in resource tables. Handy.
Code Runner.NET (not listed on the site): this allows you to run C# code as a script at the command line. SWEET!