Adjusted Earned Run Average (ERA+)
$$ {\displaystyle ERA+=100*(2-{ERA \over AVERAGE\ ERA}*{1 \over PARK\ FACTOR})} $$
ERA+ takes a player's earned run average (ERA) and normalizes it across the entire league. It accounts for external factors like ballparks and opponents. A score of 100 is league average, and 150 is 50 percent better than the league average.
The average ERA+ is set to be 100; a score above 100 indicates that the pitcher performed better than average, while below 100 indicates worse than average. For instance, imagine the average ERA in the league is 4.00: if pitcher A has an ERA of 4.00 but is pitching in a ballpark that favors hitters, his ERA+ will be over 100. Likewise, if pitcher B has an ERA of 4.00 but pitches in a ballpark favoring pitchers, then his ERA+ will be below 100.
As a result, ERA+ can be used to compare pitchers across different run environments. In the above example, while ERA will lead one to believe that both pitchers pitched at the same level due to their ERAs being equivalent, ERA+ indicates that pitcher A performed better than pitcher B.
Factoids
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Pedro MartÃnez holds the modern era (post-1901) record for highest ERA+ in a single MLB season; he posted a 1.74 ERA in 2000, which had an average ERA of 4.92, which gave MartÃnez an ERA+ of 291.
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The career Major League record for ERA+ (with a minimum of 1,000 innings pitched) is held by Mariano Rivera, a closer whose career ERA+ is 205.