Dungeondraft is a rather nice tool for drawing RPG maps. It's designed for small scale tactical maps rather than world maps. It also has an option to bring in custom assets.
Given the predominance of Fantasy assets for Dungeondraft, I've decided to try and do my own asset sets for science fiction, principally focused around Traveller. There are four asset sets at the moment, focused on Starship deck plans, starports, leisure activities and trade goods.
If you want to take a look at the sources, then they are available for free here:
All are available for free, along with the SVG and XCF files used to build the assets. By free I mean in the open source sense of the word. Use them however you want, even for commercial uses. All are currently WIP.