Monkey Cage. I was told of it because it does excellent reports of elections around the world - but it has so much more. It takes on sloppy journalism (akin to Ben Goldacre's 'Bad Science', save that it deals with poor use and understanding of political science research). It also has lots of articles ranging across the discipline of political science and related areas.
It is written in the USA - and has (save for its worldwide election coverage), an emphasis on US politics. It has an impressive list of contributors (and it was Sarah Binder's piece on the film, Lincoln [Lincoln: Best film ever about...the House of Representatives!] that persuaded me that it was a film I HAD to see.)
I subscribe my email (link on the blog). I can strongly recommend it.