Project Euler

Hey /prog/, I know I'm a little late to the party, but I was wondering how many problems you've solved thus far.
Other discussion regarding Project Euler is also welcome!

Impromptu challenge proposal:

Solve a P.U. problem of your choice.

If the chosen problem has been solved previously in the thread, your solution must exceed all previous solutions in one or more aspects of: correctness, clarity, conciseness, elimination of redundant calculation.