The logo/header for this blog was generated using POV-Ray, the Persistence of Vision Raytracer. POV-Ray is a free, multi-platform program that renders a 2D image based upon a 3D model generated from a program-like text file. I wouldn't recommend it for most purposes, as it is only a renderer - there is no export that produces a 3D model that can be used to transfer the design into another program, like an animation or gaming package.
In other words, it turns something like this:
into something like that:
A different bit generates this ringed gas giant.
But I've had experience with it in the past, and it was a useful in exercise in seeing if I could quickly generate a few planets. There are other ways this could have been done; even if I'd used POV-Ray to generate the planets, I could have composited the planets, the text, and the background in a traditional 2D image editing program. There's a lot more that can be done to make planets look more realistic, like add noise, use multiple rings with small gaps for the gas giant, etc. but they'd largely be lost in the logo-sized image. Feel free to play around with the examples below if you want to try it out yourself.
The image used for the background is a public domain image from NASA JPL, and is an amazing image in its own right. Hurrah, our tax money does provide some very pretty pictures. If you download it and place it in same directory as the source code POV-Ray should automatically find it when rendering and use it as the background.
Complete source code for the planets and the background can be found on GitHub under an MIT license. Knock yourself out if you wish. The file in question is LogoPlanets.pov.