IMO the king of the trackballs, Microsoft Trackball Explorer. I thought I had lost the ball but just found it - you can see the amount of use from how the paint
Microsoft Trackball Explorer
NeweggBusiness - Microsoft TrackBall Optical D67-00001 Black 5 Buttons 1x Wheel + 1x TrackBall USB or PS/2 TrackBall Mouse
Vand mouse'uri microsoft trackball | arhiva Okazii.ro
X-keys L-Trac Black Trackball X-keys®
Amazon.com: Microsoft Trackball Explorer : Electronics