Celestia

When you tire of looking at space, why not visit it?

Another beautiful piece of software. Celestia differentiates itself from planetarium software because it lets you travel through all of known space. You will want to see Saturn's rings (who doesn't?), but you can also travel far beyond our solar system.

Google Earth

TuxPaint

Kids love to draw and color and make a mess. Computers make the process a little less messy. Tuxpaint is a fine choice for young kids, as good as any program, free or not, of its type.

Childsplay

This suite of games is designed for children up to about ten years old. It covers pretty much all the bases of educational software for children. Math, language, puzzles of all sorts, geography, etc. Different games are appropriate for different ages, some as young as two. The graphics are great, the games are good, and the software is free. Terrific combos.

GraphCalc

The finest Windows graphing calculator ever. If you are studying trigonometry or calculus, or are just a math aficionado (I didn't want to say geek), this is it.

From the site:

".. your first, last, and only line of offense against the mathematics that threaten to push you over the brink of insanity. It slices, dices, shreds and purees functions that leave other calculators wondering what hit them."

They're telling the truth, folks. An amazing piece of software, this.