From the README:

  This is a not so short little ditty that does the simple conversion of mouse
  button presses into key presses.  By grabbing only the 4th and 5th mouse
  buttons the program is able to receive input from the Intellimouse series
  mice.  The wheel button can always be used as middle button, this program
  does not affect how the XServer reacts towards it.  A mouse with a wheel
  button is a 3 button mouse, and should be configured as such!

Note that other wheeled mice such as the Logitech MouseMan+ or TrackMan+ are
Intellimouse compatible and can be used with this as well.  For more
information on setting up your wheeled mouse to work with X, see either the
imwheel man page or the FAQ.

WWW: http://jcatki.dhs.org/imwheel/

- John
jhb@FreeBSD.org