One of my favorite procrastination sites is Sporcle. There are quite a few language-related games there so I can easily convince myself that I’m not really wasting time. Plus, I always learn something. (“Mentally stimulating diversions”, indeed!)
Watching Jeopardy the other day, I learned of another site, Guess my word. It’s really simple yet entertaining. You guess the word-of-the-day by typing a word. After each guess, you get a reply stating whether your guess comes alphabetically before or after the target word.
I’ll be adding this to my procrastination toolkit. Does anyone have other suggestions?