At a festival earlier this year, I saw Deftones, Rammstein and Tool. Probably one of the best ever festival line-ups ever. Rammstein have THE greatest stage show known to man. So much fire and explosive goodness :3 Tool are, well, Tool. They're amazing. Also seen Parkway Drive, they're great live, so much energy, and the support acts (Confession and Miss May I, among others) pull off some crazy stuff - for example, Confession got most of the standing area crowd to lie face down on the ground halfway through a song. Amazing.
Wish I had been able to see Nine Inch Nails though...