

If you're on Windows using the GIMP, there is a Voronoi diagram plugin binary that can randomly generate street-like lines ( as seen here) suitable for a city map, but it's an old plugin that isn't maintained anymore.

If you've got a small city or are drawing only a high-level view of it you can get away with hand-drawn streets, but for more complex streets like in this city map that would be mind-meltingly tedious. The hardest part of making a fantasy city map is laying down the streets-everything else (city walls, surrounding terrain, water, building fill, text, etc.) is the application of more basic cartography techniques.

However, depending on what you're going for, there are plugins that can help. You don't exactly need plugins for this sort of thing: the most important skill for using raster software like the GIMP for map-making is learning how to use the existing functionality to get the effects you're interested in.
