If you need to position an object accurately in one dimension, then a linear actuator is a good choice. Example using a stepping motor: Pololu - Stepper Motor with 28cm Lead Screw: Bipolar, 200 Steps/Rev, 42×38mm, 2.8V, 1.7 A/Phase
You can also make linear actuators quite easily using a motor (either a brushed DC motor or a stepping motor) driving a piece of threaded rod with a nut.