Average email open rates are usually published by email marketing service providers which aggregate and analyse open rates observed for their client campaigns. Thus, for big ESPs, average open rates can be computed for hundred of email marketers.
There can be significative differences between stats published by ESPs. For example Silverpop benchmark found an average open rate of 19.7% for 2012 and a benchmark report from Epsilon and the Email Institute found 27,4% for the Q4 2012. Gaps in figures can be explained by different client profiles and different practices.
Interest of open rate benchmarks is limited as open rates depend on many factors:
type of industry
type of newsletter
type of segmentation