Recently I came across the 8 mm pneumatic tubing with its various valves, quick-fit couplers, etc. They are very cheap and usable for both air and water at substantial pressure.
One of the things I could not find, however, were low-voltage valves (those I found were basaed on 24V solenoids and used some current ALWAYS when turned on. Additionally, they did not allow partial opening, which is not of much importance for me but still not cool.

So what I did was adapting a valve intended for hand usage to servo operation!
I used standard size servos (Futaba S3003 specifically), which operate at 5V, have 0.5A max current draw and are easily controlled by a microprocessor. Plus, when they reach the desired position they go idle, consuming 8mA. Cool? Cool!

1. Install the shaft coupler onto the servo
2. Unscrew the handle and cut out the walls limiting rotation
3. Fix the valve in position with zip ties
4. Insert the servo, check rotation (marks on the shaft coupler will help with determining current position), adjust the coupler position on the servo if necessary, fix in place by M3 screws.