Here's a quick trip around the internet for a great collection of various mental challenges:

* Cognitive Labs's Mind Games section features plenty of puzzles and just pure fun to strain and entertain your brain!

* The plus magazine puzzle page contains some amazing and unique challenges, such as this month's Clock Sudoku.

* Granted, Finger Frenzy 2 involves typing quickly, but the fact that you have to recall the alphabet backwards adds the mental part of the challenge. In case you're wondering what a good time for this challenge is, check out this video of the entire alphabet being typed backwards in 3.016 seconds!