Cuboro is they best toy I have seen since Lego. I recently purchased a set for my 3-year-old son, and we both have been having a blast with it ever since. The basic idea is simple: marbles and a track. The interesting thing is that the track is built out of individual wooden blocks with curves and channels cut into them, allowing the builder to create a track of whatever shape their imaginations can conceive. The marbles are moved along strictly by gravity, falling from one level to another and cutting back and forth through hidden tunnels. After starting out with some simple structures, you will quickly start to think of more complicated structures. I found the challenge to build more interesting designs quite addictive.
The company is based in Switzerland, but sets are available from several US retailers. There are several starter sets, plus add-on kits that allow you to build more complicated structures. I started with the Cuboro Standard, and recently added the Cugolino set. Although Cuboro is a bit pricey for a toy, the manufacturing quality is exceptional and you get what you pay for.