What do Americans call 12 and a half cent?

A bit. So, two bits is 25 cents.

A 'bit'

A quarter (25 cents) is often refered to as Two bits. This in turn comes from the 'pieces of eight' famed in pirate movies -- Spanish silver crowns that could be broken into eight pie-slice-shaped 'bits' to make change. Early in American history the Spanish coin was considered equal to a dollar, so each of these 'bits' was considered worth 12.5 cents

You often hear bartenders in old cowboy wild west movies ask for 2 bits (or something similar) when they have just parted with a bottle of whiskey etc...

http://www.everything2.org/index.pl?node=quarter

thanks! =)