Open Left. Today they posted their last post. It was founded by Matt Stoller and Chris Bowers about four years ago, and has since grown to embrace a variety of talented writers. There was a range of views from center-left to far left, and while I didn't always agree, I appreciated that they tended to back up their opinions with citations of fact and useful charts.
One of my favorite commentators there was Mike Lux, who will hencefrorth be posting at Crooks and Liars. But all the writers have posted their goodbyes, along with links to their future endeavors. If you stop by, now or in the future, the great conversations sparked at Open Left will still be archived there.
Best wishes to all concerned. And try to support progressive media whenever you can.