This is a derivative of Madox's Mini Centrifugal Pump, http://www.thingiverse.com/thing:4839
I simplified the design and adapted it to be printable on a makerbot without support material.
I also modified it so that the motor from an old CD-Rom drive can run the pump. These motors are all about the same size, and anyone who likes to take things apart more then likely has a few laying around.
In the photos I am powering a 6 volt motor with a 3.5 volt battery. I imagine more power would equal more water moved.
1) Print out all three parts, the Base, Impeller and Casing.
2) Drill out the center of the impeller so it fits on the motor's shaft
3) Use a knife to cut the support tower out of the center of the casing
4) Press the motor firmly into the top of the casing
5) Press the impeller down onto the motor shaft, but not all the way. Leave a little space between the top of the impeller and the base of the housing.
6) Power the motor and use sandpaper against the spinning impeller to smooth out any high spots
7) Press the base on
The real magic of this, is that if you have the motor, it takes less than a half hour to download, print, and assemble.