Trackballs That Can Spin
Here's a trackball design that spins!
True Spin Balls
As far as i know, only these 2 truly spins. I think they are using ball bearing.
Ranking of Trackball Ball Friction
From most smooth to less.
Here's ones i'm not sure how smooth they are. If you know, leave a comment please.
Trackballs No Spin
Kensington Expert Mouse