Super-fast islands in Gimp

1. New image, any size but 1024x768 works well
2. Filters->render->clouds->solid noise. Randomize seed, set detail to 10; x-size to 4ish and the y-size set to make the same ratio as the size of the image.
3. Set  the current gradient to Caribbean Blues
4. Colours->map->gradient map.
5. Done

You can extend this in various ways. For a start you could add a gradient which produces a slightly more topographic feel. If you make the width of the image 2 times wider than the height (and adjust the y-size accordingly) and turn on "tilable" you get a decent world texture.

In theory you should be able to map this onto a sphere using the Filter->Map->Map Object tool, but at the moment it seems as if that expects a square texture. In any case, you'll need to add ice caps by hand (hint: try using white to black gradients in another layer using the "Addition" or "Dodge" layer modes).

If you want to adjust the ruggedness of the map you can fiddle with the x- and y-heights before generation, or use the colour levels dialogue to modify the range and distribution of heights.

The above world map was created using the gradient file linked to here in about 10 minutes (most of which was fiddling with the poles). It's been squashed into a square so that it can be turned into a sphere

Here's the above map turned into a sphere using the Map Object function. As you can see, the polar area isn't completely satisfactory but the whole process is so quick that for a game like Traveller where the GM needs planets almost on demand, it's not bad.