When someone during a friendly hand of poker raises the bet by 'two bits' they should be throwing a quarter dollar into the pot. A bit is considered equal in value to 12 and a half cents, and derives from the very currency Americans used during and after the Revolution: good old Spanish silver 'pieces of eight'.
The American colonies couldn't mint their own coins, so used paper currency, which became useless during the Revolutionary War. The most available coins worth any value were milled 'reale de a ocho' or '8 royal' coins minted in Mexico City of almost sterling silver quality. This piece of 8 royals could be chopped up into as small as 8 times, making eight pieces of pieces of eight, each eighth possibly worth what 12 and a half cents would buy you back then (a letter to London, plus enough gin to make you bowlegged, as well as bread to soak it in).
By the end of the 18th century, the silver dollar was being minted in the US, and the currency sytem was based decimally, but the eighth and 'bit' division was still used to describe the quarter denomination coins. At the end of the 19th century, you could get a shave and a haircut for two bits. At the end of the 20th century, you could get... a call home to a friend to pick you up from the bar.
Or even give a measly raise on a bet, just for the chance to knock on wood and say, 'two bits!'