And while we're comparing artists we've seen, the following list includes all of the bands I have seen on purpose along with notable/enjoyable openers. I am sure I have seen several dozen more opening bands that were either forgotten or part of a festival lineup and I didn't really intend to see. I gave up once I hit 100:
Ace Frehley, Acetone, Afghan Whigs, Alice Donut, Alice in Chains, Allman Brothers, Amon Amarth, Anthrax, Arab Strap, Archers of Loaf, Arrested Development, Baroness, Beck, Bjork, Blind Melon, Blues Traveller, Bonnie 'Prince' Billy, Built to Spill, Butthole Surfers, Cake, Calexico, Cat Power, Circulatory System, Cloak, Colonel Claypool's Fearless Flying Frog Brigade, Cypress Hill, De La Soul, The Decemberists, Demiser, Dinosaur Jr, Dio, Disappears, Exodus, Faith No More, The Flaming Lips, Funkadelic, Funkdoobiest, Ghost, Godspeed You Black Emperor, Grateful Dead, Green Jello, Gwar, Helloween, High on Fire, Hole, Iron Maiden, Janelle Monae, Jane's Addiction, Judas Priest, King Diamond, L7, Lou Reed (not in concert, but eating in a diner), Meat Puppets, Megadeth, Mercury Rev, Metal Church, Metallica, Ministry, Modest Mouse, Mogwai, Moistboyz, Morphine, Motley Crue, Motorhead, Mr. Big, Of Montreal, Olivia Tremor Control, Oranssi Pazuzu, Overkill, Oysterhead, Pantera, Pavement, Phish, PJ Harvey, Power Trip, Preston School of Industry, Primus, Queensryche, Rage Against the Machine, Red Death, Rush, Sabaton, Sanctuary, Slayer, Smashing Pumpkins, Sonic Youth, Sound of Urchin, Spin Doctors, Spoon, Stephen Malkmus, Stone Temple Pilots, Stromae, Suicidal Tendencies, Testament, Traffic, U2, Ween, Whitesnake, Wolf Parade and Yatra
(bolded artists I have seen 3x or more)