The following list is about one of the most addictive and strange online games, here it comes…
Line Rider

Line Rider is a kind of physics toy. You start out with a blank white space and a line tool, and are encouraged to draw a diagonal line from the top left to the bottom right. What you're really doing is drawing a hill for a little scarfed figure to toboggan down. It has nice physics! And it's really addictive.
Play it here
Atomz

This game is about chain reaction. You have to make the red atom explode with the click and then make a chain reaction, destroying as many atoms as the game asks for.
Play it here
Dead tree defender

This game is so addictive! You are in a kind of fortress with two other guys and you have to shoot arrows to the incoming monsters. Incredible game!
Play it here
Dust

This weird game is a simulation game of… well, everything you see above. You use your mouse to spread the powder, water, fireworks, etc. Interact with them as if they were real. Put a bit of magma on the ground and then same nitro, the result is a kaboom!! Also you can make wind, “superballs” (bouncing balls), animated people (“fighters”), and many other things. The graphics seems ugly but it's really fun! You wouldn't mind the graphics and also it has great physics!
Play it here
And the number one is…
The impossible quiz

This game is like The Idiot Test game but really more strange. Maybe the questions don't make sense to you but they make sense for someone out there. The quiz is quite difficult but not impossible; you have to have time and patience for this “piece of strange art game”. Excellent!
Play it here and the second version “The Impossible Quiz 2” here