http://fantasticcontraption.com/ is quite possibly the best free web game I've encountered.
In it you get to solve complex (or seemingly simple looking) physics puzzles to move one item to another with only bars that you join together and motors (wheels in the game) that turn uncontrollably. It is then your job to make sense of these and make them do what you command! A very simple premise that rapidly gets out of control.
(Chris gets the blame for suggesting it to me and making it consume a huge amount of my free time today...)Permalink