NUnit to Write Unit Tests
http://www.nunit.org.
NDOC to create code Documentation
Http://ndoc.sourceforge.net
Nant to Build your solution
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 expnesss
http://royo.is-a-geek.com/regulator
.NET Reflector To Examine Assemblies
http://www.aisto.com/roeder/dotnet