Co in siding with the new release of CodeRush Xpress for C#, (give it a go today!) here is a list of FREE tools, which I think, are a must have:

.Net Visual Studio Tools

Code Rush Xpress, C# & VS2008 only- helps you code faster

IntelliSpell Community - correct you bad spelling, in your XML comments, strings etc

Ghost Doc - helps you on your XML comments

Source Out liner - provides a tree view of your source code's types and members and lets you quickly navigate to them with filtering inside the editor.

Source Control (SVN) - great article with links for plugins

.Net Tools

Reflector - Look into the compiled .Net Dll's, Awesome tool, now owned by Red-Gate

Reflector add ins - loads of plugins. worth your time to have a look.

SandBox Compare - compares the .dbproj file to the .dbproj.user, highlights the differences and allows you to merge them. (developed by your truly :D)

Web Tools

Fiddler - HTTP packet sniffer, this is a proxy tool, it lets you see the traffic between the browser and the web server.

FireFox, with Firebug

Other Utils

WinMerge - Compare 2 files/folders, highlights the Differences and merges the difference.

Expresso - allows you to test your Regex expressions, like an IDE for regex, really cool.

Agent Ransack - file search tool, it will find the file and tell you the line number of where the text appears, also has its own expression engine, and its fast.