Twitter released a new two-pane interface today featuring infinite scrolling, embedded media and an overall more information-rich experience. If you use Twitter from the web interface, it's a really nice update.
It's basically a more AJAX-y interface, meaning a faster experience, less waiting on page loads, more in-line media, and yes, something we always get behind, keyboard shortcuts. Or, as Jason from Gizmodo put it, it looks more like a Twitter app.
The update's rolling out incrementally, so if you don't see it already (chances are you don't), you should probably sometime this week. (They actually said they don't know how long it'll take, so you may have to be patient.)