NOTE: This is a popular post. Although Sizzle at one time was used in YUI 3, that is no longer the case. However, yui 3 has a very fast selector engine which uses native APIs where possible, much like sizzle. (-saw 10/06/2009)
Matt Sweeney has ported Resig’s Sizzle engine to YUI 3. This is really, really good. Sizzle aims to be a super fast selector engine. Aside from some nice design, it also uses the native selector apis when available…and normalizes them. Very fast on IE 8, Safari 3.1, FF 3.1, just plain fast otherwise.
(HT @davglass)