Mike Gunderloy has a good article about the general categories of tools that every developer should be using, I would add a couple more categories to his list:

Diagnostic Tools – Fiddler, CLR Profiler, DotTrace etc. You need to have a way to figure out what is going on when the debugger fails you.

Bug Tracking – Even if you are the only developer you still need somewhere to log bugs that you can’t fix right away. It could be something sophisticated or something simple.

Continuous Integration – This is kind of implied in the Build category, but I think it’s important enough to call it out. Whenever you check in code something should be grabbing all of your code, running all your tests, and giving you reports.

Code Generation – Your time is too valuable to bang out monotonous code, you need something to write it for you.

-James