With the latest season of 'Doctor Who' ending, you'll have the rest of the summer to catch up on your 'Who' history. If you're a purist, the best way to do that is to go back to the vaults and watch the show.
BBC America just released four more volumes, covering the First Doctor (William Hartnell, 1963-1966), the Third Doctor (Jon Pertwee, 1970-1974), and the Fourth Doctor (Tom Baker, 1974-1981).
More than two thirds of the episodes are available on DVD, and the BBC plans to eventually release everything it has. Mind you, some of the earliest episodes from the '60s and '70s were erased to make way for newer programs, but bits of those turn up here and there.
The most recent additions give a decent overview of three of the first four Doctors, and the first two decades of the show. The BBC releases the older episodes in collections representing story arcs, rather than by season, the way we're most used to seeing TV on DVD. They can be a bit pricey, but you can get a bunch of them through Netflix, and sometimes, even the local library.