I recommend you all wander over to
http://www.experimentalgameplay.com/ and take a look at the downloads there - a group of students at Carnegie Mellon University putting up the small games/toys they have created as part of their course, each one based around a single simple idea such as 'gravity' or 'sound'.
I particularly recommend Particle Suck, Gravity Head and Tower of Goo. I'll be playing Square Off as soon as someone else is in...