GAIA - Open Source Worldgen
A downloadable generator for Mac OS X
GAIA is a work-in-progress world generator I'm making for several game prototypes. My hope was to make it fairly general-purpose, but right now it's a mess of code snippets, sketches and hacks. You're very welcome to download the code and use it yourself, or simply just get the tool and generate yourself some worlds to look at.
It has some simple tools for raising and lowering water levels, hills and mountain thresholds and so on, so you can choose good parameter settings for completely random generation (or just to look at). I have future plans to add various other things. There's a source bundle here to download but if you're interested in the project you can find it on GitHub here.
GAIA was uploaded as part of #PROCJAM 2014.