The best math manipulative?

What's your favorite math manipulative?
I'm excited to get a few sets of magnetic balls (variously called BuckyBalls, Nanodots, Neocube, etc) for our student math team to explore. They're addictive to play with and can spawn discussion on a range of topics: platonic solids, crystalline structure, surface area and volume, number theory, nanotechnology, tessellations, and more that I haven't thought of. Plus, for motivation there's already a ton of videos of people creating cool things out of these. Here's a cool one to bring up recursion or fractals: