Most students these days are using TI-83 or TI-84 calculators. The TI-84 is very helpful in AP Stats. I've developed an interest in the TI-Nspire CAS as well, and I hope to use it quite often in AP Stats.
TICalc.org - A TI Calc program archive where you can find hundreds of useful programs.
The Official TI Calculator Website - You can find lots of useful information about your calculator.
A warning about graphing calculators : Many students enjoy the game features that are available on graphing calculators, and that's fine. My general policy is to assign detention to any student found using these games in class.