I spent some time early this summer playing around with Wolfram|Alpha, a computational knowledge engine. I used it when working on a lesson on measurement, scientific notation and powers of ten. I wanted to know what everyday items were about 1 cm in length, 1 meter in length, 1 km in length, etc. I had answers to these questions very quickly.
Here's a video that shows you how it works.
This is a terrific resource for teachers, but it can also be used by kids. Check out this post entitled 10 Fun Questions Kids Can Answer with Wolfram|Alpha for some great ideas.