Man this is tough. I guess it would be Devildriver back in 2004 ish? At a local club about 20mins from here. Maybe 50 people showed up. I knew most of their songs. Real cool to be up close and personal with the band. So loud. They burned up an amp or two (smelled burning electronics). Met the band afterwards. I remember this fan came up to Dez telling him he was a manager of a pizza shop and was welcome to come by for some free pizza and smoke some bud. Good times. I loved every Tool concert I've been to (seen them 6 times I think), twice with Meshuggah. Tool puts on a great all around concert with visual effects. If I did drugs, their shows would be the place to smoke it up.
I've lost count of how many concerts I've been to (venture a guess of 60+ easily). I 2nd eileen's comments about ticket stubs. I have most of mine, though I lost my Ozzfest 1998 ticket (was my first concert I went to). Some concerts, the security guards took the entire ticket. I've always wondered why.