I looked through all the water fowl pictures in Sibley and Peterson, and I couldn't figure out what this duck is. I did find a clue in Peterson, which led me to look up "domestic mallard" in google images. This led me to the 10,000 Birds blog. Charlie wrote about "Manky Mallards," and has photographs of mallards very similar to this bird. Based on his article, I'm concluding that this is a feral male mallard. (Please chime in if you agree/disagree.)
Check out Charlie's article: "Manky Mallards" (Domestic, Feral, or Just Plain Odd Mallards, published on June 28, 2007. It's fascinating. Thanks, Charlie!
[I don't know what the deal is with the little white tuft on his head. Maybe he was in a fight?]