This list contains some of the coolest developer tools.  I think the struggle will be to keep myself from installing too many of them.

Scott Hanselman's Ultimate Developer and Power Users Tool List

The first one I installed is just incredible — Notepad++.  It is a replacement for the Notepad program that comes with Windows, and does every possible thing you'd ever want to do with text files — in a tabbed interface.  Developers will especially like the colored text, which can be changed to something like 3 dozen different language varieties.

After installing the new Notepad++ program, it's pretty easy to replace the existing Windows Notepad.  First go to c:\windows\system32 and rename notepad.exe to something like notepad_old.exe.  Then, in the c:\program files\notepad++ folder, create and then run the following batch file:

copy /-y "notepad++.exe" C:\WINDOWS\System32\dllcache\notepad.exe
copy /-y "notepad++.exe" C:\WINDOWS\ServicePackFiles\i386\notepad.exe
copy /-y "notepad++.exe" C:\WINDOWS\System32\notepad.exe
copy /-y "notepad++.exe" C:\WINDOWS\notepad.exe

Voilà! Now every time you open a .TXT file, or select "View Source" from IE it opens in Notepad++.

By the way, only people who are, say, intermediate computer users or better, should attempt this.