NFL.com has a history section that covers just about every major change in NFL football from 1869 until the present! Check it: http://www.nfl.com/history/chronology/1869-1910 Stuff like "In 1909, the value of a field goal was dropped from 4 points to 3."
Nice. And, if you were not aware of it before, http://www.pro-football-reference.com has one of the most complete stats systems I've ever seen for football...