LowPro is a scripting extension to Prototype written by Dan Webb to make unobtrusive DOM Scripting much easier. Included are an onReady event, adding behaviour using CSS selectors, fixing the this reference to always refer to the element within events, and a mechanism for triggering events.
A list of the CSS Selectors supported by jQuery. It includes the regular gamut of CSS selectors as well as attribute selectors, pseudo-selectors and child selectors.