I stumbled upon The Gwigle Game a few months ago, but I forgot to blog it. It's quite clever - it gives you search results, and you have to figure out what was entered into the search bar to get those results. The more Google tricks you know, the better you do, so as the website says, it can "help you use Google more effectively". Nice!
And speaking of Google, here's a more "backwards" version. Guess what I'm searching for!