Norman Rockwell Museum in Stockbridge, MA. Ross' art is familiar to many comics fans, but the exhibit is an opportunity to see it up close at original size, and also to see his early work and influences.
I highly recommend the experience. The exhibit is at the Rockwell Museum through February 24, and I believe it will be travelling to other museums -- it's already been in Pittsburgh -- but I couldn't find a tour schedule on Ross' official site. (I also don't know if the original Rockwells will stay with the Ross exhibit.)