Google had the "20% personal projects" thing company-wide for a while. I wonder if they have some statistics that maybe proxy productivity and how they look.
> Gmail is often given as a shining example of the fruits of Google’s 20 percent time, its legendary policy of allowing engineers to divvy off part of their work hours for personal projects. Paul Buchheit, Gmail’s creator, disabused me of this notion. From the very beginning, “it was an official charge,” he says. “I was supposed to build an email thing.”