some time ago, someone on the DPS forums posted this link to DOF Master.
Online Depth of Field Calculator
Its an online calculator but if you own a windows based PC or Palm Pilot, you can download the software. Best of all its free.
You can create discs to print so that you can estimate the DOF for a given lens, aperture combo, or charts that allow you to determine HFD.
I've had great fun with it. I wish I can remember who originally posted it so that I can thank them again.
PS don't use the default settings for circle of confusion because its way off.
there are probably other online calculators too if you do a search.
Edit: Sorry Inkista, I just went through this thread again and you were the one that originally posted the link to DoF Master on page 1 of this thread...how embarrassing 
Didn't mean to repeat what was already posted.