There are several stages at Summerfest ranging in size. All but one are included with general admission. For $15 you can walk around and see as many acts on as many of the general admission stages as you want. This year we strolled in and joined the crowd to see Savoy Brown and before we left for the evening we caught a little bit of Phil Vasser. 2 years ago we saw Heart with general admission.
The largest stage (the Marcus Amphitheatre) is reserved seating (the Eagles played the Marcus in 2002). This year over the 11 day festival besides ZZT and Petty we had Tim McGraw, Eric Clapton/Roger Daltry, Justin Bierber, Usher, Rush, Carrie Underwood, The American Idols, etc.
Last night was Idol night. The Carrie Underwood on the main stage and Danny Gokey on one of the general admission stages.
One of these years, I'm going to take the week off and spend every day at the lakefront just wandering from stage to stage.