In college I wrote a little Sudoku solver for an algorithms class which was more or less brute-force with backtracking (and a little intelligence about which part was the weakest and should be brute-forced first). A fun exercise, but for better or worse, I don't play Sudoku anymore. I solved it; the slowest part of getting the computer to do it for me now is the data entry. I've lost all motivation to do it by hand.

(Online + GPL'd: http://random.fennecfoxen.org/webtoys/sudo/sudo.html )