Reflector
Reflector is a class browser, explorer, analyzer and documentation viewer for .NET.
http://www.aisto.com/roeder/dotnet/
It is particularly useful if you are having problems with a third party .NET assembly that you do not have the source for as it includes a disassembler. If you open up an assembly (File...Open) and then drill down to a class or method you can right click and select Disassemble and it will display the source code for you. There is also a dropdown on the toolbar that lets you select language so you can disassemble it in C#, Visual Basic, MC++, etc. should you so wish.
IISAdmin.NET
This is a useful little tool that let's you create multiple root websites in IIS under Windows XP which is often handy during development.
http://www.codeplex.com/iisadmin
WebLog Expert Lite
Useful free log analyzer for examining IIS log files. Gives information about Visitors, Referrers, Browsers, Errors, etc.
Xenu's Link Sleuth
Xenu's link sleuth is a website spider tool that can be useful for detecting broken links in a website or for checking what pages in a website are actually used. You might inherit a website with lots of pages but actually find that not every page is still linked up.
http://home.snafu.de/tilman/xenulink.html
