Michigan 4 times, Michigan State, Iowa, Rutgers, OSU once. Heading back to OSU this year and going to M&T for the Maryland game.. not sure if that counts. Want to get to get to all the venues at some point, but Nebraska and Wisconsin are at the top of the list. I think Northwestern would be a fun trip as well.
Been to OSU (3 times), Maryland (pre-BigTen) and Indiana. Given how much I despise this conference, I really have no desire to go on the road and see any others. On the other hand, if we played LSU, or Oregon, or USC, for example, home and home, I would seriously consider a trip.
We play at Maryland this year, so I'll be able to visit College Park and get that list down to one.
Oh wait. That's right, Maryland is broke and has no money. So they moved this game to Baltimore, to make $$$ off the 30K PSU fans who will no doubt be in attendace.