Is there a physics Puzzles site like Project Euler?
I think the Harvard physics problems of the week are pretty nice:
Link
One nice source of physics problems is books which catalog questions from graduate school qualifying exams. For example:
- University of Chicago Graduate Problems in Physics with Solutions
- Princeton Problems in Physics with Solutions
Another source of tricky problems is:
- Boston Area Undergraduate Physics Competition.
The contest is sadly extinct, but problems (and solutions!) from the contests that took place 1995 - 2004 are available online.
Just like the example student offered, this one isn't active any more, but makes a nice compilation:
Link