Knowledge Capsules | Detecting unused CSS selectors
http://www.aggiorno.com/blogs/aggiornings/post/Detecting-unused-CSS-selectors-.aspx
Detect Mobile Browser - Open source mobile phone detection
http://detectmobilebrowser.com/
The All-In-One Almost-Alphabetical No-Bullshit Guide to Detecting Everything - Dive Into HTML5
http://diveintohtml5.org/everything.html

Appendix A: All-In-One Almost-Alphabetical No-Bullshit Guide to Detecting Everything

If you want the guts of how Modernizr HTML5 CSS3 feature detection works...