The Museum of Broadcast Communications is an Illinois-based non-profit with the mission of preserving radio and television programming. Their archive includes over 25,000 television programs, 5,000 radio programs, and 12,000 commercials and some of their collection is available online. They say that over 7,000 program assets have been digitized so far. (I would imagine that they would digitize more but money and intellectual property constraints prevent it.)
To view their archive, you do need to register for a free account. Once registered, then you can search their collection. What's nice is that you can limit your search to what is available digitally so you don't have to wade through hundreds of records that you can't view.
What is available digitally runs the gamut - there are snippets from the 1950s game show $64,000 Question along with presidential debates, and "Where's the Beef" Wendy's commercials from the 1980s. The quality of the video, especially the old footage isn't the best, but it is worth taking a look.