[ALUG] Trackball for Linux?

Does anyone here use a trackball? I've been using a Kensington Expert Mouse (which is a trackball) for many years and would like something similar that will work well with Linux. Requirements are pretty simple:- Must be a trackball Must have at least three buttons, preferably four. Should be able to program buttons (e.g. my button four does double click of button one). -- Chris Green (chris@halon.org.uk)