Do-it-yourself DOF scales:

A knob-style DOF scale system:
http://www.largeformatphotography.info/dofknob/

A very nice survey of current DOF systems:
http://www.bobwheeler.com/photo/Surveys/Survey.pdf

A printable DOF scale customized to your exact requirements:
http://www.dof.pcraft.com

And lastly, a plug for a bit of software I wrote, if you have a PocketPC and you are willing to buy SprintDB Pro ($20), what I call The Photographers Assistant, does DOF calculations, among many other things:
http://handango.com/PlatformProductDetail....productId=40577