GIMP can make random world maps out of the box!
Following up on the random coastlines in this post I was going to make a GIMP script-fu *macro) to generate these automatically. Low and behold, there already are three built in to the GIMP! distribution. All of them generate coloured maps, however, and are available from the main GIMP window Xtns->Patterns->Land (shown). Also shown are the settings I used for the generated images, only changing the seed.
Note that these create an image, nothing needs to be done first. I (creatively) used seeds 0, 1, and 2 respectively.
Now to modify/extend this to draw outlines as well!