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