Good stuff listed so far and I think the Pitchfork list is fairly solid thus far as well. A couple of my favorite summer albums are 70s albums.
I can't remember the last summer that I didn't have Lynard Skynard "One ^More From the Road" or Steve Miller Bands Greatest Hits 1974-1978 in my shuffle.
A couple other obvious ones are Fleetwood Macs "Rumors" or the self titled album. Black Sabbath's "Paranoid" has withstood the test of time.
The Specials first US release was late 70s. They're a must for any Ska fan.
An obscure but great Folk/Country Album from the 70's is John Stewarts "The Complete Phoenix Concerts" - think of a Johnny Cash with better songwriting (if you could imagine that).
And throw an early Journey or Styx album in for good measure (and just to humor me).
A for the punk, start with Television and the obvious ones are the Pistols Ramones and the Clash, but don't overlook the Buzzcocks, the Damned, X, Stiff Little Fingers or The Germs. Oh, and the Jam too. A great Trio.