Charlie Wilson's War takes a pretty serious covert war (the U.S. aiding Afghanistan's 1980s-era fight against the Soviet Union) and manages to turn it into a pretty funny movie. Philip Seymour Hoffman's performance alone makes it a must-see. Roger Ebert liked it. More importantly, it currently sits atop Stacey's best-of-2007 list.
Sweeney Todd: a bloody good time. Awesome performances (not surprising, with Johnny Depp and Alan Rickman heading up the cast), awesome songs, awesome story. See Ebert's four-star review for more detail. Better yet, go out and see the movie.
I also recently caught No Country For Old Men in the theater for the second time. It's still my pick for best film of 2007.