US Weekly online has a pretty good "Celebrity Lookalikes" page, which I believe they update monthly. This month, I mostly only agree with one pair, which has been circulating for a while (and therefore is very unoriginal): Brandon Davis and Sylvester Stallone.

I highly, highly disagree with their Jessica Biel and Michelle Trachtenberg match-up; you'll see why once I post their respective doubles as seen through my eyes.

I also went through the archives (accessible on the lower right hand corner of the page) and slected some of my favorites, ones I would have never thought of but are totally right-on!