We’ve been working on a linear actuator motion system and have been experimenting with the RMCS-2207 DC Servo Motor from RoboKitsWorld.com.  It’s a really neat package, coupling together a high-torque DC motor, a gear train, an optical encoder, and a 5v serial-controllable motor controller all in one: ideal for pairing with an Arduino control system.  Code for our controller demo setup is up on github.