Best educational software

[TXlib]

IMHO, the best educational software is the type in which learning is its own reward.

My historical favorite has been Mathematica. Mathematica is a tool that solves complex problems, but it can also be used to explore many areas of mathematics.

I've recently encountered another piece that rivals it, Starry Night. It recreates the night sky, both in real time, and accelerated. You can zoom in on objects, etc. It's almost as good as viewing a perfectly dark night sky (in some ways, better.) The Pro edition is also designed as a tool, for astronomers.

Both of these are examples of software that could easily keep an intelligent kid (or adult) busy for hours, learning math and astronomy.

Anybody have other examples of software for other disciplines that falls into this category?