From Washingtonian:

R Street Institute

Downtown DC
Total staff: 62.
How flexible is it? At this libertarian and right-of-center-leaning think tank, staff get unlimited PTO (and are encouraged to use it, including two weeks in August); they also can work from wherever they need to (and might join a meeting via a screen on a mobile robot).
Other perks: $1,200 a year for wellness, $3,000 a year for student-loan repayment, $400 a year toward a new phone, and $40 a month toward cell service.