I've been messing around with Inkscape as a mapping tool, and ended up with this as a result this morning. I already have a blank image set up with the grid lines drawn into it, so this didn't take long. I started with the polygon/star tool, and set the rounded and random fields to .100 and .200 respectively. Then I drew a bunch of random polygons, randomly changing the number of corners to values between 3 and 8 as I went. Once I had a bunch of polygons I dragged them around to something that looked good, and drew a few connecting shapes with the freehand line tool to create closed paths. I tweaked the paths using the node tool and simplifying the ones I drew by hand (Path/Simplify).
Once I had something that looked good, I used Path/Union and Path/Difference to turn the collected shapes into a single path. I duplicated the path and applied Path/Outset a few times, giving me an interior path (the caves) and an exterior path (the shading). I applied Filters/Distort/Pixel smear to the caves, and Filters/Scatter/Air spray to the shading. Tweaked down fill alpha a bit and played with colors a little, then exported to png. After a little cropping with Gimp, here it is:
