Fantastic Contraption is a great flash-based game. It allows you great flexibility in the design of your contraption. The goal of the game is explained in the tutorial - essentially all you're trying to do is get the pink ball into the pink area. Sounds simple enough, right? Each level gives you a different challenge. Either some pitfall or obstacle is in your way.
Many of you have probably already played, but I'd like to see your contraptions in the comments section of this post.