Mouse encoders (or Optical encoders) allow the use of a trackball or spinner and offer the functionality of a mouse hack in an easy-to-use form. They offer several benefits including ease in connecting controls and time savings.
The Opti-PAC allows for easily connecting optical-type arcade control devices including trackballs and spinners to a PC.
Relevant Link: Ultimarc's Opti-PAC Page
GroovyGameGear's Opti-Wiz Page
Relevant Link: Hagstrom's ME4 Page
It's possible to perform a simple hack on a ball-mouse that allows it to connect to a standard arcade trackball (and perhaps spinners?). This is done by removing the mouse's optical receivers and wiring the trackball's encoders to them.
Relevant Link: How to turn a Mouse into an Arcade Trackball Interface (PDF) Relevant Link: Build you own Spinner Wiki Entry