Logbook Pro and Logbook Pro PDA Companion are great. The PDA companion comes for Palm or pocket PC, and works on a regular PDA or a smart phone. Plus it will track expenses and such, and you can set up any extra columns you feel you need.
Enter your flight in your toy of choice and sync it later with Logbook Pro in your favorite computer. berry simple, berry easy Joe
I keep mine synced on my desktop, laptop and PDA, and backed up on HD, Thumbdrive every few days and on CD at least once every few weeks.
I also keep a paper log, because some folks still want to see the pen and ink version.