A somewhat rare specimen of US currency: The presidential dollar coin (this one featuring Abraham Lincoln). Now, rare isn't quite right: Dollar coins are all over the place, just nobody seems to like them. Quite the pity, considering how nice these coins look! Admittedly though, they are so much heavier and bulkier than dollar bills. -- So whenever you see someone paying with "Honest Abe" or any of the other presidents, they have most likely been using some sort of vending machine recently, which are most of the time incapable of returning bills as change.
Fun fact: The presidential dollar coin program is backed by a law that among other things codifies a long-standing rule: That a presidential coin is not going to be made featuring a president who is still alive. Perhaps it would be a little too Roman Emperor otherwise.