These are some of my most used applications; I tend to drag them around with me on a USB drive every time I go to a new client site - kind of like my application comfort zone

This is by no means an exhaustive list:

• The .Net Reflector - http://www.aisto.com/roeder/dotnet/
• NUnit Test Framework - http://nunit.org/
• NUnitForms (NUnit extension for WinForms testing) - http://nunitforms.sourceforge.net/
• TestDriven.NET (VS.NET testing addin) - http://www.testdriven.net/
  
• NCover (Code Coverage) - http://ncover.org/
• NCover Explorer (see what you missed testing) - http://www.kiwidude.com/blog/
• SharpReader (RSS Aggregator) - http://sharpreader.net/

Most used framework!

• The Castle Project - http://castleproject.org/ (especially active record and mono rail)