Since I have a new job, I had to sit down at a fresh installation of Windows today and get my machine developer-ready. I already miss the Dock and Transmit and the Terminal, but I'll deal.

UPDATE: Today (7/14) I sat down to a new MacBook Pro and had to repeat the process... hooray!

One thing I noticed was my reflex-like action to go download 4 5 plugins no web developer should be without. They are, in no particular order:

1. Measure It! How wide is that sidebar? Don't pull out the DOM inspector or Firebug, just MeasureIt! I also like that once you drag the box it persists on the screen and you can drag it around to compare measurements.
2. ColorZilla Sample any color in your browser - don't open Photoshop, just hover over a color you like and voila! Also generates Photoshop pelletes, but I don't open Photoshop, so I don't use that feature.
3. FireBug I can't actually tell you how awesome this is. Being able to see the http response of every file that was loaded and how long each file took to load is already a killer app, not to mention dead-simple editing of any markup, CSS or JavaScript on your page, and being able to execute JavaScript on a live website without having to open any windows. FireFox 3 people, make sure you get 1.2 beta.
4. YSlow extension for Firebug - a plugin that has plugins? This tool can help you analyze the bottlenecks in your page optimization, giving you an A-F score for facets of your page load (very useful for high-traffic sites).
5. Web Developer Toolbar My #1 used feature is the Resize menu. So many pages break my window from being exactly 1024 wide, and I also want to check things out at 800 wide as well. This just feeds the my OCD streak and lets me get on without worrying.

These don't include any of my plugins for personal productivity, web browsing, media sharing, or Search Engine Optimization. Instead, these are 5 plugins I think should come pre-installed on every developer's machine.

Also, if you're doing Drupal development, I hear very good things about this Theme Developer Module for Drupal 6. I'm not using 6 on any production sites yet, but I think it will prevent you from having to open up TextMate and do a "Find in Project".

Leave your favorite development plugins, or other kinds below.