For my TomCopter project (a 12-cm quadcopter where the circuit board doubles as the frame), the hardest part to source is propellers. There are lots of plastic propellers available for RC aircraft, but very few of them come in both left- and right-handed versions, and very few are as small as I need (< 73mm diameter). The best I had found online were "Air Hogs MINI STORM LAUNCHER Propeller" replacements on eBay. They were too small, though, and were designed as pushers, so I had to drill through the hub and mount them backwards.
Then I found OpenSCAD, which is a script-based CAD system. I'd tried to design propellers before in Blender and several GUI-based CAD programs, but I wouldn't be able to iterate -- once I finished the propeller, if I decided I needed a different diameter or airfoil pitch or something, it'd be very hard to tweak. With OpenSCAD, I was able to make all the magic numbers tunable, so I can make a propeller of any shape and size just by hitting "compile".
There are some limitations of the language, unfortunately. The extrude is not as flexible as I'd like (I can't parameterize it), so I had to build it out of short extruded segments. You can see the stairstepping in the 3d model. Also, since there isn't a way to append to arrays, I can't specify the airfoil cross section the way I'd like. (For this experiment I went with a very simple linear cross section.) Finally, since I had to take the union of hundreds of short extrudes, it ended up very slow to compile.
Shapeways did the 3D printing. (I also looked at Ponoko, but Shapeways was cheaper.) I had it made in several materials, but "White, Strong, and Flexible" worked best - more on that later. Shapeways was also a little behind on shipping it to me, and accidentally polished one of the sets, but no biggie. The best part: since the propellers are so small, they are actually cheaper than most molded-plastic ones online! Only $4.06 per set.
Next up, some tests to see how they performed...