I do most of my mapping in QGIS and GRASS on Linux. Perhaps not the best tools for from-scratch world-building, but there's a lot of capability there. Certainly the ability to readily move between scales is nice - though I have been known to overstress my PC with too much detail. Presentation perhaps doesn't do justice to the "under the hood" work put in.

The purpose is to support my involvement in a Nations roleplaying forum.

Currently I'm working on a planet. The goals are Earth-like, plate-tectonic feasibility, detailed and realistic topography, and a degree of randomness. I've got as far as the plate boundaries and the rough outlines of the continents. Next is the "large scale" topography, which then gets the small-scale detail added to it. Then I sort out the rivers and lakes, then the climate, then things like the national borders and main towns, then the main roads.

I have a couple of pipe-dreams, that probably won't happen. One is a Google Maps style interface to my planet; doable, but not a high priority. The other is a complete simulation of planetary geology, something that's a tall order even if I radically simplify the chemistry.

PS: Where's the best place to ask about map projections?