Unlike W3Schools which shows what browsers support (or don't support) a particular method or property, without explicitly specifying the version...
...this new Reference shows which browsers support each of the Objects, Functions, Properties, Statements, Operators, Comments, Directives. Only the latest versions of popular browsers are taken into account & surprisingly Opera is excluded.
The complete source (15.2MB) of this HTML5 JavaScript Reference application is available for download & review.
Update(14/Jan/11): If you work with JavaScript, also get this bookmarklet - Steve Souders has created a meta-bookmarklet that includes DOM Monster & a few other performance analysis tools available as bookmarklets. His meta-bookmarklet works with mobile & desktop browsers.