The Internet Archive hosts a massive, massive collection of live concert recordings in MP3 format - all completely legal, uploaded by consent of the bands. Any recommendations for particularly good stuff here? Internet Archive: Digital Library of Free & Borrowable Books, Movies, Music & Wayback Machine
Ever heard of Country Joe McDonald? There are several great performances by Country Joe in the Internet Archive. Try this one. If you don’t like it, then you probably just don’t like Country Joe. If you do like it, there are 13 other recordings here.
Anything by the Grateful Dead or Little Feat.
There’s a buttload of Live Tenacious D Concert Recordings that are quite good. Tenacious D is Jack Black and Kyle Gass.