Here are some great live albums that seem to have slipped through the cracks;
The Who: "Live at Leeds". I've always felt the best live albums make you feel like you are right in the audience. The sound is far from pristine, like "Ya Ya's", but for excitiement, energy and spirit, it's hard to beat. Check out the re-master with extra tracks.
"Woodstock": Some low points, but more than enough "high" points to make this a classic. Standout performances by Ten Years After, SLy and the Family Stone and Hendrix
"Frampton Comes Aive": Yeah I know..too commercial and popular, but still a landmark album. Great performance, supportive crowd and some good rock and roll to boot.
"Slade Alive:" Not well enough known here in North America but this British rock/glam/pub quartet could be the best live act ever in England. Great versions of "Born to be Wild" and "Darling Be Home Soon", despite the belch in the middle!