Egg City Radio is what became of the great Post-Punk Junk blog [previously]. And it's still a treasure trove if you're looking for shared out-of-print albums and live sets--not only from Post-Punk but also many other genres as well. [ECR previously (via), -er, -est]
Maybe you'd be interested in full, downloadable live concerts by Iggy Pop and David Bowie (1977), P. J. Harvey (1992), Neko Case (2006), or the Arcade Fire (2008)? The online, L.A.-based music publication Web in Front hosts a terrific collection of concert podcasts from rock bands and songsmiths of every era. From Talking Heads (1979) to Pavement and Dinosaur Jr. (1994); from dour troubadours like Lou Reed (1976) to dour troubadours like Nick Cave (1998), it's an inexhaustible trove. (Recent podcasts.)
The Ultimate Bootleg Experience is an ecclectic mp3 blog dedicated to live boots with a good number of posts up, and the links in the archives seem to stay live for quite a while.
The "Bootlegs" section over at The Rawking Refuses To Stop has a ton of great stuff, from a stripped down live acoustic Jeff Tweedy & Jay Bennet set (right before YHF was released) to a mock-up version of the unreleased new Radiohead album using the best available live tracks from the most recent tour to a rare Paul McCartney, John Lennon & Stevie Wonder jam session to all sorts of smaller gems.
Podbop. Completely stupid name. Not a bad idea. Enter your city, and it will come back with MP3 links and podcasts from bands soon to play in your town, in a "try before you buy tickets" kind of way. Works quite well for the US, other countries look like they need updating.
Live releases a free downloadable MP3 in what is sure to be a long line of overwrought and sappy tribute songs inspired by the tragic events of yesterday.