These are the things I install on every single frickin' Windows box I build (and yes, I know, but you're wrong, that won't work for me):
Firefox (with del.icio.us and Googlepedia)
Google and Wikipedia search providers for IE7
Note that Process Explorer is not quite as much of an absolute requirement on Vista as it is on XP/2k, thanks to the new Resource Monitor, which is hiding behind an innocuous button on Vista's Task Manager Performance tab. Also, cool new thing from Russinovich is Process Monitor, but I'm still very comfortable with PE and so only check out PM from time to time.
Also - this is a good toolbox list.