[Alas, this turned out to be too good to be true. I don't know when it happened, but the free download has been taken away.]
This comes as a total surprise to me, but Led Zeppelin's "Stairway to Heaven" can currently be downloaded for free from last.fm. All of the free downloads that I've ever seen on last.fm have been approved by the artist, so I am assuming that that is the case here; however, another "song" called "Led Zeppelin is the worst band ever" (which is actually about half a minute of someone banging on a piano), which claims to be a Led Zeppelin song, also showed up today. I don't know if this means that last.fm has been hacked, or someone is playing a bizarre joke, or... well, I just don't know. All I do know is that one of the most iconic rock songs ever made can currently be downloaded for free from a site known for legitimate music downloads.
I may as well keep this post short, as I really don't know how long this song will remain available. If it isn't already in your collection and you haven't gotten your fill of it from listening to classic rock radio, go check it out. Happy listening!