Not 100% passive, but fantasy sports has been a nice side income for me this year. I used to play a lot for almost nothing, but then found daily fantasy games (pick a new team every game day, play against other people) and have made a decent return on my investment.

I play on fanduel.com almost exclusively. If you have any questions or want some help getting started, send me an email (in profile). My referral link if you're so inclined: http://www.fanduel.com/?invitedby=yudarvish&cnl=da