Rubik's Cube at Cool Math Games: This is a computer version of a Rubik's Cube. You can twist it all around to check for your next moves.
Bridge Crossing at Cool Math Games: This is a classic logic puzzle. I get lots of emails about this one from classes that can't solve it. I promise you - there IS a solution.
Wolf, Sheep, Cabbage at Cool Math Games: This one's another classic. You have to get a wolf, a sheep and a head of cabbage across the river... so that nothing (or no one) gets eaten in the process.