There have been dozens. Just off the top of my head, Gary Hart, Arnold Schwarzenegger, Jim McGreevey, and Bill Clinton have been involved in scandals as politicians, and Chris Brown and Rihanna is one celebrity example.
In mid-2011, it was revealed that Arnold Schwarzenegger engaged in an extra-marital affair some ten years ago and has been supporting the child that resulted. A similar story broke a few years earlier regarding John Edwards. The Sandra Bullock-Jesse James break-up in 2010 consumed the media.