Not a philosopher, but a moral entrepeneur and leader, more so than most philosophers.  The Times obituary is fairly informative about his long career, including his survival of political persecution.  It doesn't say so directly, but it is notable to have lived more than 90 years and to have been on the right side of almost every major moral and political issue of the day.  Here's one of his many lovely songs that I remember seeing him sing in concert back in the late 1970s.

UPDATE:  A nice Democracy Now memorial program on Seeger.  (Thanks to David Auerbach for the pointer.)