Here's a list of FOSS tools and frameworks that you may find useful when developing .NET software. I've looked at, played with, or used all of these tools over the last few years.
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)