I believe the answer is Tim Keefe, who had a 0.86 ERA in 1880. If you're looking for modern history, it would be Greg Maddux, with a season ERA of 1.56 in 1994.
Bob Gibson of the St Louis Cardinals went won 22 games against 9 losses with a 1.12 era in 1968. The following year, the mound was lowered. In 1985 Dwight Gooden won 24 games against only 4 losses, 268 strikeouts, 16 complete games and a 1.53 era for one of Baseball's most statistically dominant seasons. The redoubtable Greg Maddux follows closely behind.
Dutch Leonard of the Boston Red Sox in 1914 had an ERA of 1.01, thus being the lowest ERA in modern history. tc
ERA records are questionable, since it depends on the amount of pitching that the pitcher actually performs. Actually, Dennis Eckersley had a 0.61 ERA in 1990. He pitched 63 games, but only 73.1 innings. But 73 innings is enough to establish respectable stats. Nine runs in 73 innings is quite respectable.
In 1982 Rickey Henderson set the single season steals record at 130 for the modern ball era, besting Lou Brock's previous record of 118. During his record breaking season he stole home not once, but twice!
Tony Esposito, who holds the modern era record for most shutouts in a season (15), achieved his shutout record as a rookie. Though he had played 13 games the season previous, he was still considered a rookie and won the Calder Trophy, the NHL's rookie of the year award.
Tim Keefe of the 1880 Troy Trojans had a 0.86 ERA in 105 innings and is generally considered as having the lowest ERA in an MLB season as the 1880 season was only 84 games long. Since 1900, the lowest ERA belongs to Dutch Leonard of the 1914 Boston Red Sox at 0.96. He had a 19-5 record and threw 224 2/3 innings.
The major league pitcher with the highest ERA in a single season who pitched 100 or more innings was Bill Chamberlain. He recorded an ERA of 7.74 in the 1893 season while playing for the St. Louis Browns.
In 1876, St. Louis had an ERA of 1.22 in 64 games. Since 1900, the record is held by the 1907 Chicago Cubs with 1.73 in 155 games.
The club record is 1.69 accomplished by Nolan Ryan in the 1981 season.
If you're talking about the Modern Era (1900 to present) then it would be Roger Maris.
Duch Leonard with a .96 era.
Tom Seaver's lowest ERA in a single season was 1.76 in 1971. Seaver had a record of 20-10 with the New York Mets.
The all time MLB record is 680 innings by Will White of the 1879 Cincinnati Reds. White pitched 75 complete games that season and had a record of 43-31 with a 1.99 ERA.
In 1982 Rickey Henderson set the single season steals record at 130 for the modern ball era, besting Lou Brock's previous record of 118. During his record breaking season he stole home not once, but twice!
1898 Boston Beaneaters with 62 ... their record was 62-15.
In 1914 Dutch Leonard had a 0.96 ERA
Jeff Gordon won 13 races in 1998. The last person to win 13, was Richard Petty in 1975.
Tony Esposito, who holds the modern era record for most shutouts in a season (15), achieved his shutout record as a rookie. Though he had played 13 games the season previous, he was still considered a rookie and won the Calder Trophy, the NHL's rookie of the year award.
Mariano Rivera's lowest ERA in a single season was 1.38 in 2005.
Through the 2009 season, for starting pitchers that pitched at least 162 innings and qualified for the ERA title, that was Allan Anderson with a 2.45 ERA in 1988.