Here’s one that Nate Silver on BP calls Quik ERA:
QERA =(2.69 + K%*(-3.4) + BB%*3.88 + GB%*(-0.66))^2
Silver explains as follows:
“I call this toy QuikERA (QERA), which estimates what a pitcher’s ERA should be based solely on his strikeout rate, walk rate, and GB/FB ratio. These three components—K rate, BB rate, GB/FB—stabilize very quickly, and they have the strongest predictive relationship with a pitcher’s ERA going forward. What’s more, they are not very dependent on park effects, allowing us to make reasonable comparisons of pitchers across different teams….
Note that everything ends up expressed in terms of percentages: strikeouts per opponent plate appearance, walks per opponent plate appearance, and groundballs as a percentage of all balls hit into play. In the 2006 season, Andy Pettitte, for example, had a 19.6% K rate, a 7.9% BB rate, and a 62.7% GB rate, giving him a QERA of 3.68. Note further that QERA is exponential, which is appropriate since run scoring is not linear.”