We've internally created a "scoreboard" for motivating developers to review each other's code.
Would love to get this upstream if possible.

Attached diff works against 1.6.11, applied to latest git code, but untested against latest git code.

There may be some weirdness with accuracy, eg. a user who's stopped contributing never has their statistics updated, and shows up high on the list even though obviously hasn't done any reviews in the past month/2weeks, but generally gives a mostly/directionally correct stats summary, at least.