These are slightly font-centric.
Slick little bookmarklet that figures out what fonts are being used on the page you are on. I have already used it several times.
A Firebug (for Firefox) extension that “counts all the fonts on a page and orders them by color and size before happily outputting some json”. You can read more on what it does on their github page.
An open source web font generator for Mac OS X.
I haven’t really played with this enough, but I can see it’s value.
A super simple guide to responsive design.