Single Season: 0.96, Dutch Leonard (1914)
Single Season (Live-Ball Era): 1.12, Bob Gibson (1968) (pictured)
Career: 1.82, Ed Walsh (1904-17)
Career (Live-Ball Era): 2.22, Mariano Rivera (1995 - Active)
Smallest ERA not in Hall of Fame: 2.38, Eddie Cicotte (1905-1920)
Ed Walsh is probably the least known player on this list, but his 1.82 career ERA is nothing short of remarkable. He spent 13 seasons with the Chicago White Sox while ending his career with a one-year stint playing for the Boston Braves.
Although Mariano Rivera is a reliever, the fact that he's pitched nearly 1,200 innings in his career makes him worthy of mention.
Bob Gibson put together one of the greatest pitching seasons in MLB history in 1968, tossing 13 shutouts while compiling a 258 ERA+.
Eddie Cicotte was a part of the 1919 Chicago Black Sox squad who threw the World Series, otherwise he would undoubtedly be a Hall of Famer.