For a few months I've been working on-and-off on a program called mkplanet. It's a reasonably simple script that glues together two different programs to create realistic-looking imaginary worlds.
I have been trying - without success - to create an online version so that people wouldn't have to install the program, as it requires jumping through hoops that only professional IT people are likely to be able to do. Unfortunately, it's proven more complicated than my time and access to resources (specifically a dedicated virtual server) allows, so I'm just going to release the program so that those who are interested can give it a try.
I will post again when I've uploaded it, but in the meantime here are some samples of the program's output. At the moment the most well-tested mode creates Earth-like temperate planets, but With a bit of tweaking it can be made to create rocky and cratered worlds like Mars and Venus, ice worlds like Hoth, and many others. It's just a question of defining a set of colours that are used for different altitude levels.