We require our students to get TI-83+ or TI-84+ graphing calculators for use in chemistry and then require them for use in pretty much all science courses after that. I end up using them a lot in Physics. We mainly use them for graphical interpretation of lab data and/or with a variety of probeware from Vernier (I'll talk about that later).
I was just checking out some of the great new apps that are available now. There are ones for archiving notes, creating and using flash cards, and ones for creating and using interactive timelines. These all have application outside of science. If your school requires a TI-graphing calculator you might give these things a look to see if you can use them. I'm going to give the flash card program, StudyCards, a good look to see if I can use it to help students memorize formulas and symbols in electronics.
I also like to get the kids programming or modifying programs on the calculators. These calculators are really mini-computers. The TI-83+ has more memory than the first computer my family owned (Apple IIe) and probably more processing power. I get students writing programs to solve the Pythagorean theorem and for Vector Components (both of which are very similar). I've found they get a much better grasp of how to solve problems if they can write a program to do it.
Often students just go through the motions when solving problems. Writing a program forces them to stop and think about what they are actually doing. Typically, after I show them how easy it is, I get a few students in each class who get excited about programming and write programs to solve problems for other classes. I don't know how their math teachers feel, but I think it's great.
Here are some links for programming the TI83/84 calculators: