UniBrows is an IE add-on that runs the complete IE6 rendering engine and web-application platform inside an IE8 tab.
Now, this might seem odd at first (why on earth would one want to take a few steps backwards?), but it essentially is a giant leap forward as it will allow some giant corporations to upgrade to newer IE’s whilst maintaining compatibility with some old websites (mainly intranet-sites) which they can force to be run in an IE6-frame …