A Google recruiter, while doing his spiel, told me in a light-hearted, offhand manner that some engineers call it "120% time". Having lurked here long enough and read similar sentiments from ex- and current google employees, it wasn't really news to me, but I was pleasantly surprised at his candor.
But do any of the other companies listed do 20% time? AFAIK, they only have infrequent "hack days".
Yeah, Google still strikes me as probably a great place to work. 20% time is not common in the industry, and trust me-- if you work for Google you work in a much better place than your typical "enterprise software" hell-hole.
But this might affect their ability to attract absolutely top-tier talent.
Ours instead does one week a year. They get scheduled like any other project. It means they can't be as ambitious or unbounded, but it also provides uninterrupted time to work on whatever-it-is you are interested in.
But do any of the other companies listed do 20% time? AFAIK, they only have infrequent "hack days".