RobinWords is a vocabulary building Time-Waster
RobinWords is a simple, non-Flash game. It starts you off with one four-letter word (in the screenshot, the game picked "mist").
You then need to come up with another valid four-letter word by changing just one of the letters in the word that the computer provided. You can't shuffle the sequence, either. So, in this case, I changed "mist" to "gist."
Then, it's the computer's turn, and it has to do the same thing that you just did. Turning "gist" into "gift" isn't very hard, so the computer got this one right and bounced the game right back to me.
If the computer uses a word that you're unfamiliar with, you can click the word to go directly to its definition at Dictionary.com (not my favorite dictionary website, but it works).
This is a simple, no-frills brain teaser, and it can be quite fun to play if you like words. It's very nice and fast, and they say a multiplayer version is coming soon – I'm looking forward to that.