LeoCAD
Do you (or your kids) enjoy building things in lego but never have enough bricks? Well, now there's LeoCAD, a FREE lego building application with infinite bricks and quite a few different varieties.
The application is available for both Windows and Linux environments and while it's fairly easy to use, though probably suitable for 10 years and over. Not only can you build things in it, but you can also animate them.

You can download LeoCAD from the following site
http://leocad.org/
Lego MiniMizer
If you're looking for something that's a bit better suited to younger children - or you want something that's instant fun for yourself, then the online minimizer is the way to go.
http://www.reasonablyclever.com/mini/kidsafe.htm
You can design your own lego people using this system... It's fast and easy and works entirely on the web (so it should work on Mac, Linux and Windows).
Here's one of my people who started out as being Smurfette;






