Just thought I would share my techniques for building custom worlds. Might make a tutorial on the wiki if people are interested. Why settle for random when you can play God and create a world exactly the way you want it? Desert goes here, major river goes there; mountains, hills, swamps: you decide. And the best part is that all the tools are free.
GIMP
http://www.gimp.org/ is an open source image editing program. I use it to draw the outlines of my continent or island, white on a black background. Start with a 1028x1028 pixel file, it gives you more control and you can scale it down to 257x257 (the size used by Perfect World) later. after I the use the levels command to set the white point to something quite low, around 80 or so. This is your sea level land. Next I like to make mountains and hills. I usually make a separate layer for these features. You can draw them in manually or use the selection tool to draw an outline. Then use shape filled gradient to draw mountains that slope nicely from their peaks to the plains. I like to use the fingerpaint tool to drag some ridge spurs out from the main mountain, and give more interesting topology to the coasts. There are really an endless number of techniques you can use to create your basic land mass.
After you have some topography drawn in GIMP, save the file as a .png. Then use the excellent free height map editor, Wilbur
http://www.ridgenet.net/~jslayton/software.html, to perform some tweaks and pre-erode your map. Wilbur has two erosion tools, a "precipiton" based system that simulates rainfall washing soil downhill, and an "incise flow" tool that calculates water flows for the whole map, and erodes rivers based on flow rates. When using the incise flow tool, be sure to use the "fill basins" tool, every single time. I like to run a few passes of the precipiton tool, fill basins, and then run a few passes of the incise tool, using gradually increasing gradients and decreasing widths. This creates river valleys that are wide near the mouth and narrower near the source.
If your rivers aren't going where you want them to, you can use Wilbur's hill and valley tools to get them to go where you like. On one map, for instance, I created a large, high plateau where all the water drained off the plateau on one side. This created a major river feeding into another major river over a HUGE waterfall. I will often switch back and forth between GIMP and Wilbur in order to use the different tools each provides. The curves and levels tools in GIMP are very useful for fine tuning your contours, using curves for instance you can create continental shelves, flat topped mesas, and sharp spiky peaks.
After you have your map looking as you want it to, it is time to bring it all together in Perfect World
http://dffd.wimbli.com/file.php?id=2354. First, scale your image down to 257x257, then save it as a bitmap (.bmp) file. Import your bitmap into Perfect World, then use the various tabs to calculate rainfall, drainage, volcanism and savagery. Import your world gen file, set your map generation parameters, export your world gen file and give it a test in dwarf fortress!
