What about an omni wheel but instead of having a bunch of wheels around the main one, you just have a bunch of ball bearings stuck into it.
I'm not sure if something like this exists yet, I don't see why it shouldn't.
I see a problem with this though. If you were able to push an omni wheel in any direction, then it wouldn't be able to provide any traction when you rotated it with a motor. It would just spin in place.
Now, someone mentioned something about "the wheels on the bottom of teacher's desks."
I assume that you meant something like these:
http://www.photoflex.com/photoflex/p...300/caster.jpg
Those are called "casters", just for future reference.
EDIT: I just saw something about limiting the casters' rotation as so that they don't allow the wheel to spin in place. Ignore the second paragraph in this post.