This is a simple adjustable-volume straw pipette. It should be very useful around any wet lab and works nicely with a relatively high range of possible volume settings. You can calibrate the volume but it is generally only good for low precision work. The pipette has two thumb nuts for adjusting upper and lower stops.
To complete the build you will also need a spring (Pez dispenser) but it may work without as the membrane is elastic, a kids balloon, drinking straw, and some tape. Good luck!
See a demo: http://youtu.be/N2Ax4OGokGM
Print the upper body, lower body, plunger, and two thumb gears. You will need a spring, which I got from a kids Pez dispenser for its form factor. The balloon membrane is elastic so you may get away without the spring but it is nicer to have it. You will also need a balloon, some tape, and a straw. It is designed for one particular straw diameter but I can upload others if there is demand. See the attached photos for assembly steps. I printed all the pieces at 0.1mm with 15% fill and 100% fill for the plunger.