I'm sure you've heard of this by now, but just in case... Wolfgang's Vault is the company that owns and markets the archives of the late Bill Graham (the ultimate rock promoter). They've just opened the Concert Vault - which features 300 full concerts, all of which are 100% free to stream!!
A sampling of the available concerts: David Bowie at Nassau Coliseum in 1976, The Cure at the Ontario Theatre in 1984, Elvis Costello at the Philly Spectrum in 1984, and Hendrix at Winterland in 1968.
A quick check on the forums gives the following little tidbit:
In the next 60 days we will add new functionality to the Concert Vault. For example, listeners will soon be able to create and share their own playlists from our live performances. In the next 90 to 120 days we intend to offer full-concert downloads and, in 180 days we will present never-before-seen live performance video from the thousands of concert videos we have in our archive.
Check out Concert Vault.