Got the "Lectricalc" and "Conduit Fill" and find them very useful. Plan on adding the others.
I'm working up a estimating spredsheet so that I can quote a price on-the-spot for small jobs, ones I usually do w/o a written contract.
I figure it will do a couple of things--make sure that I don't forget something and underprice a job, yet allow me to quote a price on the spot while leaving a more professional impression with the client than if I just said, "well, lemme see, that's 6 can lights, a three way switch circuit...that'll be $WHATEVER."
In fact, many of my clients use PDAs and I think it'll be real cool to "beam" an estimate from my PDA to theirs...or at least me business card.