DeFish is simply used to defish images taken with a fisheye lens and convert them to rectilinear or cylindrical projection. Hence the name. One of it's goals was to demonstrate usage of technology already known before fisheye patents were "invented".
Read about this software and download it at Ken Turkowski's site (version 2.2 as of 3 June 2003; carbonized: works on both OS 9 and OS X).
A ReadMe and Help file comes with the program but you will find a tutorial and more information at Ken's DeFish Tutorial page.