Summer 2005 - Summerfest, saw Good Charlotte headlining the Marcus, with Hoobastank opening, and also the tail end of Evanescence. (I was young, give me a break. )
Summer 2006 - Summerfest, saw Kid Rock headlining the Marcus (can't remember the opening act, save that it was good), and left about halfway through when he started doing overblown solos on every instrument on stage, went and saw Buddy Guy instead (AWESOME, especially when Kid Rock came on and did a duet after his show and Buddy Guy out smarmed him).
Summer 2007 - Summerfest, Weird Al Yankovic. My favorite concert to date.
Summer 2008 - Summerfest, Matisyahu. A fun concert, lots of weed smoke floating around, and amusingness with Jewish reggae (it was his birthday and they bought out a cake with candles on it, but Orthodox Jews don't blow out candles. Whoops....).