With Microsoft's release of ProcessMonitor, the next version of SysInternal's FileMon, I thought I'd share the 5 free utilities I use most often in my day to day programming.
In no particular order (and I realize there's nothing original in my list):
Reflector is a "class browser", which pretty much it let's you look inside .NET assemblies and figure out how they work. Recently I used Reflector to uncover a [possible] threading bug in MySQL Connector/NET. I also looked inside the SynchronizedQueue class to see exactly how it worked (yes, it was obvious but I wanted to make sure).
Fiddler is an HTTP Proxy aimed at helping you debug HTTP requests. It's particularly useful at Fuel because we work with a lot of different data protocols over HTTP. Of the 5 tools, it's probably the one I'd be most screwed without
FileMon is one of those "last resort" tools – you don't use it often, but when it gets to the point where you need it, it comes in really handy. FileMon monitors file/directory access, so it's great when you think their might be permission issues or missing files.
Lately I've been using SC less often, but it's still pretty handy. SC is a mini-IDE which lets you quickly compile C#/VB.NET code and run it as a console application. It's great for testing small chunks (snippets) of code.
Visual Source Safe
I know a lot of developers hate visual source safe. For the most part I agree with all of you. The truth though is that any source control is better than none (ya ya, not if it corrupts your repository). If you can, consider buying the relatively cheap SourceGear Vault or installing the free Subversion, but we are using VSS here at work and it's doing it's job just fine.
I haven't had a lot of opportunity to work with TDD.NET lately (doing a lot of planning/and very small projects). I love tools that enhance VS.NET, and this is the best one if you're into unit testing.