Writer for The Nation, the Huffington Post, Alternet, TAPPED, Salon, Washington Monthly, and a research fellow at the Media Matters for America. Also makes videos; see this for a description. For a 2005 issue, see this.
For an example of how Blumenthal can't seem to write a sentence without lying, see his Tom Tancredo smear which was eviscerated here. That just covers the lies in the first page of a two-pager; I got tired.
In addition to lying, he engages in outlandish race-baiting, such as of a Toby Keith song, or his attempts to smear Sean Hannity, or Samuel Alito. In regards to the latter, instead of going after the man himself, he went after his son.
For another smear and yet another example of his bad reporting, he smeared Sarah Palin using guilt-by-association.
And, there's also his strong support for illegal immigration as evidenced through his feverish attempts to uncover the "anti-immigrant" movement. He's gone as far as supporting high school kids walking out of class in a race-based display of solidarity in support of illegal activity: huffingtonpost.com/max-blumenthal/walkout_b_18016.html
Meanwhile, from his side of the fence, Christopher Hitchens called him a "young skunk who hasn't learned to piss yet" and Marc Cooper called him a "featherweight tyro and designated puncher for enraged but impotent Democrats".