I have browsed a lot of open courseware in physics and have never seen any university or other affiliation offering certification of completion. I believe that it is a somewhat controversial subject right now. However, as sights like Khan Academy and MIT OpencourseWare are becoming more interactive, connecting global communities of viewers of select courses, that is becoming more feasible. One institution commented within an MIT course discussion page I was reading, asking several questions to the viewers about their perspective on certification and what they would require for it to be worthwhile. So, although I don't think you will be able to find certification right now, from my point of view it seems like a very current issue and we will have to see what happens in the future.