Very good collection of tools to choose from for developers, they are all time savers and productivity helpers.

A tool I can't do without is the HTML Tidy Validator extension for Firefox:

   http://users.skynet.be/mgueury/mozilla/

You have already included W3C "Validators" links in your list (good), but I recommend to have this Firefox extension for local validation (including accessibility).

This will help produce higher quality pages and maybe help with SEO/SERP etc...