For the past few years, Scott Hanselman has been posting his list of Ultimate Developer tools on his blog. If you are into .Net development, it is definitely worth a look. Check out http://www.hanselman.com/tools.
Some of the tools he mentions are tools that I’ve started using over the past year. Others are tools I plan on using very soon. My favourite tools to date are Fiddler, Web Developer for Firefox, and WatIn Test Recorder. Most of the items on the list are freeware or open source and none are available as part of the standard Microsoft Suite of controls.