Oh man, i've been to so many great ones...here's a few memorable ones:

-311 20th anniversary show w/ Incubus (plus about 4 other times)
-Slipknot (last year's Jaegermeister tour)
-Avenged Sevenfold (twice)
-The first Family Values Tour (Korn, Limp Bizkit, Orgy, Incubus, Rammstein)

I don't know if any of ya'll are from the DC/Baltimore area, but one of the local radio stations used to have an all day event called the HFStival (WHFS was the radio station). I went to about 5 consecutive HFStivals where i saw: Offspring, RHCP, Linkin Park, Sevendust, Staind, Fuel, Jimmie's CHicken Shack, Green Day, Stabbing Westward, Live, Papa Roach, POD, Cypress Hill, Third Eye Blind, Blink 182....That's just naming a few :p . Man, those were the days...