Game type: browser-based, user-submitted puzzles
Time demand: 3 minutes for each. How long you spend on it overall is up to you, and your one-more-turn-itis.
What it is: Sixteen terms are given to you in a random (I believe) order, and these terms can be sorted into four groups of four terms each. Your job is to click on four at a time to try to get them in the proper groups, and once you're down to the last eight terms, you have only three more tries before you lose. Oh, and you have a time limit of three minutes for the puzzle altogether.
The trick, of course, is that many of the groups have overlapping terms, or even groups that seem like they should be there, but aren't. So while on first glance some groups may seem easy enough to sort out, you could be in for a surprise.
For me, half of the fun lies in creating new puzzles. I've done two so far, and am musing over more possibilities...
Anyhow, this is fun for some quick playing here and there, so go try it out if you like puzzles at all.