I'm currently dusting off and documenting an old project of mine - a set of simulation tools for conditions on a fictional exoplanet. It can answer the question how climate on Earth changes of you replace the Sun by a White Dwarf, or if you tilt Earth's axis by 60 degrees, or if you put Earth on Mars orbit.
It can even do the last stable orbit around a black hole, and in a simple implementation it allows to compute the Greenhouse effect.
I've used it quite a lot for SciFi story writing to get the physics of my worlds right and to assess how close to a black hole a spaceship could possibly go without running out of fuel.
The code is now public, it's GPL and a tutorial series is growing.
http://www.science-and-fiction.org/scie ... ilder.html
Enjoy!