HTML5 context menu API:
http://www.whatwg.org/specs/web-apps/current-work/multipage/...
I've seen custom contextmenus generated on pages[0] that override the built-in context menu but for some reason I've yet to come across a page[1] that adds to the existing context menu until today.
[0] replacing example: http://developer.yahoo.com/yui/examples/menu/tablecontextmen...
[1] appending example: https://bug617528.bugzilla.mozilla.org/attachment.cgi?id=554...
the reason is likely because browser compatibility for the context menu js api is pretty minimal [1]. firefox can use it on pages that are only going to be displayed in firefox, but for the internet at large it's pretty useless.