I already have a pair of Logitech mice (Revolution MX, MX310 aren't bad in themselves) but wouldn't mind recommendations on a good trackball that isn't in the Topre/uTron realm of cost and isn't some shoddy knockoff.
Only other preferences being towards multiple/remappable buttons, good DPI range, and being relatively common (i.e. it's not as rare as an M13).
Even though it's not a trackball per se, I would be open to the IBM Spaceball.
Interface isn't an issue (read: PS/2, USB, RS232 are fine).