I had a couple of Capsela kits when I was a kid (which explains a lot, I know), so this is the kind of things that tickles the geekiest and most obessive part of me:

Siftables, developed at the MIT Media Lab, are little bricks with little monochrome screens and a little wifi you can put together to literally build applications. How sci-fi! I wonder how many years until you can buy these at Toys'R'Us.

On the other hand you can buy the littleBits easy electronic kits right now, but it’s like comparing a MITS Altair against a ZX Spectrum. I’ll pass the Altair and wait for the Speccy.