Personally I am a big fan of MachDice. It's a $0.99 dice rolling app that supports complex dice pools, holding/rerolling individual dice as needed, and has a One Roll Engine mode which is unique among dice rolling apps to my knowledge. The only thing I notice that is lacking is a success-counting mode for things like WoD and Shadowrun, but that's not hard to do visually.
The other thing I've done is to bookmark relevant pages from the d20PFSRD.com site and put them on the dashboard for my ipod. Quick and easy way to look up relevant info for free.