Liquid flowing through a long pipe can carry a lot of momentum, a lot of energy, due to the huge mass of all the liquid in the pipe. (How does its total kinetic energy compare to other things humans put motion in a controlled fashion, e.g., rockets, oil tankers, trains?) Can this momentum or energy be used for something interesting? You can't extract the full energy due to friction in the pipe.
When the flow is stopped, the momentum causes an effect called a water hammer (assuming the liquid is water). One device that harnesses the water hammer effect is the hydraulic ram pump. Any others?
One needs to be careful so that the energy is harnessed to do useful work, not just burst the pipe at it weakest point. Making the pipe very thick along its whole length is prohibitively expensive.
Inspired by a story of repairing a large oil pipeline under Los Angeles.
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