This site is the homepage on my browser. Between MMA blogs and Wikipedia, I pretty much just use the web for politics (and, of course, the occasional deluded rant) - and the three sites I use are this one, HuffPo - and Talking Points Memo.
Now, it's not the most exciting format in the world, and it's not a teeming fungal mass of progressive action (yeah, I think of DK as a hunk of mushrooms), but they do some great investigative work. For instance, here is them dismissing the ACORN bullshit by reporting on a meeting in July between ACORN and state officials to address any and all issues that is not disputed by anyone.
And, of course, there is the occasional smart ass nugget of gold.
Best. Headline. Ever.