If compared to common XY-coordinate input & push buttons, it's relatively new control: majority of older devices were not equipped with wheel, ring or drum. As it became a part of standard UI controls and must-have functionality, many vendors started to add it to the improved models replacing former ones.
The results were not always great: attempts to find a place for scrollwheel on existing case produced many devices where scrolling operation was simply inconvenient, due to relocation of the palm or unnatural bending of fingers required. Most optimal HW thing, perhaps, is a scrollring (Sony, Kensington, Cursor Controls): the bezel surrounding the ball is instantly accessible and quite convenient.
Another approach is, to use the ball itself for scrolling, i.e. convert common positioning signals to the scroll commands. On some trackballs, dedicated button was used for that function (ITAC Mouse-Trak and Evolution, Fellowes OptiTrack). Rarely, third dimension of ball movement - twisting, was involved (Tektronix, Kensington) in generation of Z-axis movement signal.
To enable scrolling for devices without any hardware/firmware support, that mode could be achieved purely in software, via manufacturer's driver if particular button is pressed (Logitech Autoscroll etc). Also, various software utilities were created by independent developers, assigning property of scroll-mode switch to one of the buttons. In fact, such scrolling method is often more convenient than dedicated wheel: most of time, I prefer it even if using device with 'real' hardware wheel present. As the ball and primary buttons are always under fingers, this method does not require any additional efforts and is totally seamless.
My personal preference is excellent WheelBall utility by Kamigaki:
http://www.kamigaki.net/wheelball.htm
Very versatile and highly configurable software, working flawlessly under Windows 7 x64. Unfortunately, it's japanese-language only. The author has stopped developing or supporting it, so I've modified the binary executable - replaced Japanese text with English translations in settings (except for the tab labels) and right-click menu, and added version information to the executable. Japanese help file was used as a base for making English PDF with some useful links added.
Recompiled version is available here:
https://trackballs.eu/media/software/Wh ... 200Eng.zip