A piece of history

In 1975 HP released a slimmed down version of its original pocket calculator (the HP-35), and called it the HP-21:

This bad boy sports four stack levels of Reverse Polish Notation calculating goodness. And, since there is only one line on the display, you have to keep the other three levels of the stack in your head in order to use them. Original List Price: $125

(I gave this one to a friend, because I wanted to make sure that it found a home with someone who could appreciate its retro-awesomeness)