DSLRUsers.com

Go here to find out what it does:

http://www.gogebic-pc.com/dofcalc/WhatItDoes.html

Go here to see all program options:

http://www.gogebic-pc.com/dofcalc/options.html

For the program downloads, go here:

http://www.gogebic-pc.com/dofcalc/

You'd think he woud have added a box for the dof( far - near = dof )which is the 'useful' bit....

He wants money for it too. Not that I am that cheap but there are some good free alternatives around.
This one is great http://www.dofmaster.com/custom.html It provides templates for making your own DOF scales for use in the field where carting a computer around is not practical. This is probably the best that I have seen for actual usability where it counts.
It is also very easy to knock up a spread sheet (as simple or complex as you like) to do the job. Here is one that I did a while back: http://www.pixspot.com/albums/userpics/ ... ulator.xls

Cheers