I stumbled on Paul Irish and Divya Manian's HTML5 Boilerplate after looking for a solution to fall back on a local jQuery library if Google's CDN ever went south, and reading about how to use this technique for WordPress. Ben Kutil also helped me undersand how the fallback technique works a little bit, and I tested and implemented on Balsamiq's site.
HTML5 Boilerplate is not a library, but a template and set of awesome examples for how to get started integrating HTML 5 into your sites. They provide techniques for future-proofing HTML/CSS/JS, with cross-browser normalization, performance optimizations, even optional features like cross-domain Ajax and Flash in mind. They also give you a starter apache .htaccess config file with caching rules and prep your site to serve HTML5 video, use @font-face, and use gzip.
Check out HTML5 Boilerplate. You can also watch a screencast of Paul Irish demonstrating it on Nettuts.