Some of my favorites:
Droplet and the Water Cycle--answer a question, then go through a maze.
Lunar Lander-- Use gravity and inertia to land on the moon, earth, or Jupiter.
Mars Lander--Same as above but with better graphics.
Meteor Madness--Use your mouse and hold down the button to go further up.
Write Like an Egyptian--your name in Egyptian hieroglyphs.
X-2000 Mini Maze--Most challenging at the highest level.
Your Weight on Another Planet
Game sites with many games:
Creation Games--a lot of random games and activities.
Gamequarium Science Games -- links to several subjects full of games.
Kids Domain Science Games--a long list of thinking games.
Magic School Bus Games
NASA Kids Games--games and puzzles.
NASA Science games--some require downloading.
Sports Online games--all require Shockwave or Flash plug-ins.
Spacers Fun Zone--space games.