Computer controlled water valve
If you're in the US, try a 24 V DC solenoid valve from McMaster-Carr: http://www.mcmaster.com/#solenoid-valves/=8to0so

To control it from a computer, you'll also need a 5 V relay that can be controlled by something like an Arduino and a 24 V power supply. You might be able to find a solenoid valve with a low-voltage relay built in, but I've not seen one before.