Note that Visual Studio Team suite covers much the same functionality as some of the tools listed below and the there are a number of other commercial tools listed at the very end that may be worth considering.
Here we go...
Visual Studio Tools
CoolComnands - Handy add-in for VS2005
TestDriven.NET - Commercial use requires a license
UnitRun - run unit tests from within VS2005
Build Tools & Version Control
CruiseControl.NET - Continuous Integration build server
NAnt - An alternate to MSBuild
SubVersion - Enterprise level version control.
TortoiseSVN - Shell extension for interacting with subversion repositories
NUnit - the only real choice for unit testing in .NET
RhinoMocks - mock object framework
TypeMock - an alternate mock object framework
Watin - Web testing in NUnit
Fiddler - HTTP Debugging Proxy
Firebug - Firefox addin that lets you view & make live changes to pages
Charles - HTTP debugging proxy, including speed throttling
NHibernate - save and retrieve objects from a database without writing SQL code (plus a whole lot more!)
ActiveRecord - implementation of the Active Record pattern (uses NHibernate)
NDoc - for .NET 1.1
SandCastle - Microsoft's documentation generator. Here's an article on integrating with CC.NET
Gemini - Effective Bug Tracking system (written with ASP.NET & NHibernate). License required for commercial use.
Gemini Subversion Integration - Log Subversion commits against a Gemini issue
Jira - Comprehensive bug tracking system (in Java)
FxCop - Static code analysis (design rules)
SourceMonitor - Static code analysis (cyclomatic complexity, etc)
NCover - Code coverage. Use with NCoverExplorer in CruiseControl.NET
Reflector - Lutz Roeder's indispensable utility
Notepad++ - a great notepad replacement, includes syntax highlighting
Castle Project - Inversion of Control (dependency injection) containers, MonoRail and more
log4net - Logging framework
Aspect# - Aspect oriented programming for .NET
CopySourceAsHTML - copy VS2005 source as HTML. Great when blogging :-)
Paint.NET - You're not still using MSPaint, are you?
Re-Sharper - VS2005 productivity enhancer. A must-have!
CodeSmith - the de-facto standard in code generation.
Mercury - Very expensive, but the best test tools you'll find going around by far.
dotTrace - code profiling (performance & memory analysis)
Ants Profiler - code profiling (performance & memory analysis)