Bandera Roja en Linea (BREL) is an excellent leftist website. I would say it's one of the best leftist websites I've seen. It is from the Movimiento Socialista de Trabajadores (MST) - one of the groups in the Puerto Rican Frente Socialista (Socialist Front). It has excellent content, is updated frequently, and is very professional looking and done using PHP. I'm impressed.
We Weren't All Wrong by Alex Young, on the Democratic Underground site, is a great response to CIA Iraq expert David Kay's famous quote that "It turns out we were all wrong" about WMD in Iraq. As this article points out, literally almost the entire world got it right; the only ones who got it wrong were the few war criminals surrounding Bush and Blair.
Dean's Death from BlackCommentator.com is an article not touting the merits of Howard Dean, but exposing and explaining the corporate media's role in killing any politics that are considered outside the confines of what the rich want to exist.
I just found this right-wing blog that focuses on the communist left. It is called commiewatch and is run by a guy who claims he used to be a leftist but now has become a rightist. Although the rightist politics are annoying, the blog is interesting because he reviews articles from a bunch of different U.S. left newspapers every week. I keep finding myself checking it out...