Peter has a post giving an overview of some tools that are essential to him as a developer.
I would say that I use most of them but not all and I would like to add a few to the list because i mainly do webdevelopment and that's a different story then winforms.
Obviously I made the choice to go for The asp.net 2.0 atlas framework so I won't sum up any other ajax frameworks. That is reserved for a separate post.
Internet Explorer :
Fiddler - a tool to view http traffic (that gets sent through IE)
WebdevelopmentHelper BHO - A tool written by Nikhil Kothari and I consider it crucial
Internet Explorer Developer Toolbar - This has got a lot of cool things you do want to know about your page sometimes.
Greasemonkey - makes firefox highly extensible.
WebDeveloper toolbar - This has got a lot of cool things you do want to know about your page sometimes.
Greasemonkey Http Trace script - does the same as the BHO or fiddler for IE.
We are mostly using team foundation server to do continuous integration. However sometimes when a lot of different parties are involved we can't use the team foundation testing suite and then we use other tools to do our integration.
TestDriven.NET - does nunit testing and code coverage in VS 2005 - this used to be free but it isn't anymore.
CruiseControl.NET - does the builds based on nAnt or msbuild build scripts etc. It's the continous integration server alternative that we use
GhostDoc - essential for documenting your code.