I know that some bands officially sanction concert bootlegs (notably the Grateful Dead). Others tolerate it unofficially and some make them available commercially (e.g. The Music site sells soundboard recordings from Peter Gabriel and The Who). I wish more artists made official bootlegs of their concerts available. I would much rather pay for a recording of a concert as a memento than pay a ridiculous amount for a rip-off tour programme or an official t-shirt.
Nugs. There are concerts from a bucket load of bands (including The Black Crowes, Kings of Leon and Metallica) for download that can be purchased from Nugs.net. However, they also have a stash of free downloads.
The band I want to feature here is Gov't Mule who currently have seven concerts available for free download from Nugs. My daughter's guitar teacher used to have a sheet outside his shop that claimed the difference between rock musicians and blues musicians was that rock musicians try to perform an exact replica of their album tracks every night while blues musicians try to make every night unique. While they have a southern rock feel, Gov't Mule probably falls into the latter category!
I haven't downloaded all the concerts but the first one I went for was the Beacon Theater shows from the 30th and 31st of December 2002. It should give you a good idea of what they are all about.
Let me know if you have any other recommendations from the Nugs free stash.