Google search + Google Chrome browser have this implemented, when you browse the google search results using your up and down arrows (only possible in Chrome browser) and than press CTRL+ENTER it opens the search result selected in a new tab but without focus to that tab. You can now use up and down keys in the same list of search results and CTRL-ENTER more results in new tabs to visit later on.
Does anyone have any idea how this is done? As it only works with Google Chrome at the moment I suspect something is added to the browser as well but I can't find anything on this subject.