The Makers Workbench shares this project which measures the rate of liquid flow using a Seeedstudio Water Flow Sensor and Seeeduino.
This is part of their Arduino Controlled PC Liquid Cooling project. The flow sensor uses a simple rotating wheel that pulses a hall effect sensor. By reading these pulses the Arduino mathematically calculates the liquid flow with a 3% accuracy rate and sends the reading via serial to the PC. Schematic and Arduino source code included.