LEGO has recently come out with a new linear actuator piece as part of their new Power Function sets. The new actuators have a movement range of 5 studs, or about 1.6 in (4cm), which correspond to 26 full revolutions of the input shaft. The actuators also have a built in clutch mechanism to prevent damage from over-extension. Klocki has more pictures of the actuators. This may be a useful alternative to much more expensive linear actuators, like the ones from Firgelli.