New York Knickerbockers base ball club, circa 1847.
"The Knickerbocker Club was formed by members of the Gotham Base Ball Club, which became the first organized club in NY in 1837. Although the Knickerbockers existed in 1842, the Knickerbocker Club officially formed on September 23, 1845. The rules they recorded that day and were most likely used for a number of years prior became the foundation for the "New York Game." As the this set of rules was further developed, its popularity eclipsed the Massachusetts Game and the Philadelphia Game, which was based on town ball, to become the preferred game for amateur clubs and subsequently professional baseball.
There's some dispute between the men above over who actually came up with and/or assisted in the writing of the original rules of "base ball." (which is to be expected when epic beards are involved).
Read more about each of the players at 19cbaseball.