Buy a palm pilot - lots of software options. At the previous job I downloaded a Free flight-planning software package called "Copilot" that also did a SUPER w/b. Program in a few aircraft factors and it will give you all the usual numbers for takeoff and landing, plus has a little graphic W/B envelope like the POH that shows you where you are loaded in the envelope.
I'm working for a place that has Ultranav (for jets) which apparently costs around $1000 for a desktop/palm package.
BTW, I got my palm pilot on ebay for $17.