the Kensington TurboBall (if you can still find one) has four programmable buttons, and works great on my OS X Panther G4 notebook.
I've been using this thing from system to system for years..
No jumping, and its bullet proof.
I noticed that the extremely smooth surface became less sensitive to movement after having "unwashed (oil-laden) hands" so I sanded the ball with a very very light sandpaper, and then added some WD-40 and then washed the excess off. Now the surface of the ball is flatter (non-shiny) and works flawlessly.
GS US Technical Support Team Leader
P-C2D 1.86GHz/2GB w/ 256MB GeForce 8600GTS; G4 OS 10.4.10 1GHz/1GB "[that other software] is a fancy set of electronic marker pens"