The HP 12C is my favorite calculator that I've ever owned. I LOVE the touch of the HP keys, I LOVE reverse polish notation (RPN), I love the simplicity. The HP 12C has just about every function you need and very few function you don't need. It is also the perfect size. It's a pretty standard tool so I feel a bit silly writing about it, but just in case you don't have one, I decided that maybe I should blog about it.
First of all, RPN makes more sense once you get used to it than any other way of talking to a calculator. You hit the keys the way you actually think of equations. For instance, to calculate 1 / ( (2 + 3) * 3) on a normal calculator you would do something like 2 + 3 = x 3 MC M+ 1 / MR = probably. With the HP you would do something like 2 ENTER 3 + 3 x [1/x]. In other words, enter 2, take 3 and add it, take another 3 and multiply it, then invert it. You sort have have to get used to it, but trust me. It's more natural.
The HP 12C also has a bunch of very useful financial functions for dummies like me. You can listen to an entrepreneur give a pitch and punch in... 5 years, projections for the first 5 years, starting cash, then pow you get expected cash at the end of 5 years and pow, you've got your internal rate of return (IRR). Or if you're buying a house, punch the interest rate, how much down payment, how many years, how often you pay, and pow, you've got your monthly payments. You can do date calculations and some simple programing too.
Anyway, I can't live without it and I have software on my palm and my PowerBook that emulate the HC 12C.