Suppose you've got an object floating freely in space. You have a vector you want this object to point towards, and a vector representing the direction it's currently facing. From these two, you can get the rotation (matrix, quaternion, whatever) that represents the change in orientation to bring the two vectors into alignment.

If you only have the ability to apply torque (derivative of angular velocity) to your object, what's a good algorithm for applying torque over time that won't over/undershoot the destination?

(In this case, it's a space ship that wants to automatically orient itself in the direction of travel using thrusters. Roll is irrelevant.)