If you like puzzles, you should check out Merriam Webster's Online Daily Crossword Puzzle! It's really cool, and definitely challenging and fun. The interface is intuitive and it's interactive you can reveal and/or check letters, words or even the entire puzzle as you work through it and it has a timer (which you can pause when necessary).
It has the ability to save your progress as well. I started a puzzle earlier today, and before I began to write this post I went back and it brought me to the game I had already started. Really nice feature. Get your brain ready, the timer is ready to start ticking... Be forewarned, it can be addictive and it's easy to while away the hours playing.
You'll need Flash 6+ to play (it's ubiquitous these days, except on Linux).
» M-W.com Online Crossword Puzzle
Be sure to bring your brain!
» M-W.com Home Page
Dictionary and much more...