i'd go with wilbur to start with. or perhaps use some stochastic method (generate noise, threshold to create shoreline.. can be done very quickly) in gimp et c.

mine does world maps at 4096 * 2048, so not so cool for regional maps. still doesn't do more than render a height field atm (added bmp output to the version i think i linked, but that's all..)

there was another guy here with the same intent.. an app to generate the lot, nations, cities, roads, for people who want arbitrary planets.. his last blog entry is 4/10..
http://generated-worlds.blogspot.com/