NUnit to write unit tests
http://www.nunit.org.

NDoc to create code documentation
http://ndoc.sourceforge.net

NAnt to build your solutions
http://nant.sourceforge.net

CodeSmith to generate code
http://www.ericjsmith.net/codesmith

FxCop to police your code
http://www.gotdotnet.com/team/fxcop

Snippet Compiler to compile small bits of code
http://www.sliver.com/dotnet/SnippetCompiler

Two different switcher tools, the ASP.NET Version Switcher and the Visual Studio .NET Project Converter
http://www.codeproject.com/macro/vsconvert.asp

Regulator to build regular expressions
http://royo.is-a-geek.com/regulator

.NET Reflector to examine assemblies
http://www.aisto.com/roeder/dotnet