I have used several palm based applications including D-Plan (a gradient factor model), Palm VPM (VPM-A model), M-Plan, J-Plan (another gradient factor model) and Dive Calc (VPM-B model).
At this point I normally use D-plan but I have adjusted the gradient factors a bit to approximate the slightly more conservative VPM-B profiles. Dive Calc however has lots of promise and Ray Little is planning to release a new version toward the end of the month with new features and improvements that will make it a very attractive alternative to D-Plan.
D-Plan is about $60.00, While Ray's Dive Calc is $25.00 and the rest are freeware.