The Expert Mouse is a big optical trackball made by Kensington. Here are some photos of it. As usual, drivers are provided only for Windows and MacOS.
As this is essentially a mouse, there is no problem in using its basic point-and-click functionnalities with Linux. Moreover, the Scroll Ring can be configured as if it was a scrollwheel. But it is a bit harder to make the fourth button work without disturbing the Scroll Ring use. Here is a working configuration:
In the configuration file of the X server, you need a section which look like that:
Section "InputDevice"
Identifier "Mouse1"
Driver "mouse"
Option "Protocol" "Auto"
Option "Device" "/dev/input/mice" # put here the path to your device (system dependant)
Option "ZAxisMapping" "5 6"
Option "Buttons" "6"
EndSection
This alone will make all your buttons usable on X, however applications are using buttons 4 and 5 for scrolling, not 5 and 6... And if you put "4 5" as a parameter of "ZAxisMapping", then you lose the fourth button... The solution is to use xmodmap to remap the order of the buttons. You can do that for example by putting in a file which will be passed to xmodmap when loading X (for example /etc/X11/Xmodmap):
pointer = 1 2 3 6 4 5
That's all! Although this was tried only with Linux and Xorg, it is probable that it will work with other Unices too.
While I'm speaking of this trackball, let me say that its use is very comfortable. There is a short adaptation time if you have only used standard mice before, but the learning curve is very smooth. At use, if long mouse session are sometime painful for you arm and your wrist, this trackball is far better than a mouse as you don't have to move your wrist each time. The only drawback of this thing is its price.