Microsoft seems like the last one to the new browser party that's dragged out over the last month, but they are the first to make their entrance final with Internet Explorer 8.
One cool UI bit, which you can see in the screencap, is that tabs are colour-coded, so you can see where your trains of thought spring from—all of my Giz-originating tabs are one colour, while all of the links I opened up in tabs from Fark are a different colour, and so on. Pretty handy.
Accelerators are its version of add-ons in a way, or at least that's how they tout in them. In practice, they let you quickly jump to web apps, like Google Maps (or their own Live Maps) or translate something from Japanese or whatever language via their own service, or one of your choosing, like Google Translate. So IE8 is way more open in how you use embedded services—you're not stuck with Microsoft's own, which is good since a majority of the browsing public will be using this one day (probably, anyway). But without features like Firefox's add-ons and extensions, Chrome's rawer minimalism, or Safari/Opera's Top Sites, it still feels a few steps behind the rest of the pack.