I keep 2 at arms reach:

HP 48S

This one I've had for almost 10 years. It used to have programs and games, but they got lost after the batteries died. With 32k of memory, I kept it full to the max during college, but now days I forgot how to do most of the fancy stuff. I mostly use it for regular calculating and its built in conversions menu is very nice. It cost me $150 and I bought it used. Just a month later they came out with the G model for $120. I kicked myself hard for that one.

The nice thing about this one, is you enter numbers into the stack, and then do your function on it from there. Its hard for people to get used to, so after borrowing it once they never ask to borrow it again. :)

This one is a godsend in the Architectural world. It allows you to input feet, inches, fraction (to the sixteenths) and even all your normal calculator functions with no conversion to decimal. It also has slope solvers which come in real handy. For $30 at Costco it was a steal.

-Steve G.