We have been playing Notpron here at work. It literally fries your brain!!
If you like a challenge and thinking outside of the box, I suggest you check it out!
Notpron - The Hardest Riddle Available on the Internet
It gets harder and harder as you move through the levels. Try not to cheat and let me know how you get on
If people want to work together solving the puzzles then post the level and any progress you have made.