Free D.O.F. calculator (Windows)
 

DOF calculator is a free program for Microsoft Windows XP and Vista, that allows the user to calculate the depth of field and hyperfocal distance for several lens settings (focal length, f-stop) and subject distances. It supports dSLR cameras with APS-C (crop factor 1.5x and 1.6x), APS-H (crop factor 1.3x) and Full-Frame (crop factor 1.0x) sensor sizes.

Focus Perfect

My favorite calculator
 

Here's my favorite web calculator:

Julian's Lens Calculator

And here's another Windows DOF calculator that also calculates background blur separately:

Calculating Image blur - a Multipurpose Optical Calculator