Kensington Expert Mouse Trackball
As a gamer (first-person shooter), I believe a trackball is a superior interface to the mouse. I searched for good trackballs, and everything had only a little tiny ball for the thumb. Then I found the Kensington Expert Mouse Trackball. It has a large ball, about the size of a billiard ball, raised up in the center. It is USB, and has four buttons and a scroll wheel.
The four buttons were a bit of an issue. The main problem was that the "middle" mouse button, used in X11 applications for pasting text, was in the upper left corner -- the hardest button to reach. I wanted it in the upper right corner. The scroll wheel wasn't working quite as intended, either.
I start X with xdm, so I put the following command in /usr/X11R6/lib/X11/xdm/Xsetup_0. If you use startx, you need to put this in your xinitrc (either the global or your private .xinitrc). In other words, it needs to be executed whenever the X server is started. The command re-arranges the order of the mouse buttons.
xmodmap -e "pointer = 1 6 3 2 4 5"
In my XF86Config, my mouse configuration looks like this. The device is the USB mouse device. The ZAxisMapping makes the scroll wheel work, but buttons 5 and 6 are dependant on the xmodmap reordering above (for normal mice, buttons 4 and 5 are typical).
Section "InputDevice" Identifier "Mouse0" Driver "mouse" Option "Protocol" "auto" Option "Device" "/dev/ums0" Option "Buttons" "6" Option "Emulate3Buttons" "off" Option "ZAxisMapping" "5 6" EndSection