Instaworld

Here's your chance to play God.

Terragen is a small but very powerful terrain generator that allows you to create stunningly realistic landscapes. Here is Terragen's image gallery so you can browse what other people have created.

You can control whether you want lakes and rivers, whether the atmosphere is clear or hazy, how much cloud cover and what types of clouds, direction, size and brightness of the sun, and the list goes on. There are dozens of variables you can tweak, and best of all, it's free. And at under 3MB, surprisingly compact. (Windows, Mac versions)

Once you play around with creating landscapes, you can check out Campath, which allows you to create MPEG animations of "fly-throughs" over your world.