A "JavaScript utility that emulates CSS3 pseudo-classes and attribute selectors in Internet Explorer 6-8."
css3
Getting to Work with CSS3 Power Tools
A compilation of the more interesting attributes available in CSS3 and how they can be used in modern browsers.
HTML5 Boilerplate
As noted on the site, this is "the professional badass's base HTML/CSS/JS template for a fast, robust and future-proof site" While I likely won't use this whole, there are some very interesting pieces to this worth using, especially in the style sheet and .htaccess files. I'll likely appropriate many of the concepts for my work.
This is an outstanding contribution to the Web development community by Paul Irish and Divya Manian as well as all of the people whose work they built upon.