Open Street Map is a wonderful tool. One might say it an opensource clone of Google Maps. It has a built in editor and a permissive use license. There is a stand alone editor called JOSM that allows bulk edits. Interestingly enough you can use JOSM to create maps from scratch.
I play a weekly Dungeon & Dragons game. The dungeon master has created his own world and has provided sketches of the various continents, and some additional detail for the city-state we are playing in. This got me to thinking about taking those sketches and turning them into a OSM style tile map. Using JOSM (plus a plugin) I was able to import the sketches, scale them, and outline the landmasses, add in some roads, rivers, lakes, etc. It was interesting, but far from graphically pleasing.
Some more research turned up a product called TileMill that creates good looking map tiles. There is some middleware needed to import the JOSM data, but I managed to create my own set of custom tiles and post them on my groups website.
I haven’t quite gotten the hang of the city detail. TileMill seems to crash if I try to generate all 22 zoom levels. By default TileMill stores the tiles in a single MBTile format. Perhaps the file is just too big for my computer to manage. We are talking about 580,000+ tiles!