Gives you many details on running processes. Including IP Connections etc. Although these functions are really nice, the "Find Handle or DLL" functionality is a real life saver.
Every Developer gets the error message "can not access file because it is used by another process" once in a while.
But which programm/process is it? This is where the "Find Handle or DLL" functionality enters the stage. Simply enter the filename, and Process Explorer will tell you who is sitting on your file.
Fiddler is a HTTP Debugging Proxy which logs all HTTP traffic between your computer and the Internet.
Lutz Roeder's Reflector
Excellent Tool for .NET Assembly reverse-engineering. Many plug-Ins available
Subversion & TortoiseSVN & Ankhsvn
Subversion is a versioning system and TortoiseSVN is the fitting Subversion explorer client, implemented as a windows shell extension.
Ankhsvn makes these 2 a dream team as in brings a Visual Studio 2005 & 2008 plugin, that allows you to handle most tasks directly from within Visual Studio.
Once you understood the concept of versioning and Subversion you never want to live without it. Even for standalone developers it makes perfectly sense to use it. You need no server component, simply create your repository on your disc or filesystem, import in your stuff, checkout and never worry again about versions or unlucky modifications.