The biggest is the "Sziget" (Island) Fesztivál, in every august. It is held in northern-Budapest, in a big island of the Danube. Every year more than 1000 performances!
The full capacity is 70 000 visitors per day.
They started in 1993, with a quite small festival, but it grew up to one of the biggest rock festivals of the year. So, next year is the 20th year!
Hammerfall, The Killers, Korn, Placebo, Snoop Dog, Gotan Project, Within Temptation, Motörhead, Judas Priest, Prince, The Prodigy, Faithless, Muse, Al Di Meola, Placebo, Buena Visa Social Club, Fatboy Slom, The Offspring, Iron Maiden, Jamiroquai, Sinéad O'Connor... etc.
In 2009, they had 290 000 visitors in one week. In 2011, Sziget was ranked one of the 5 best festivals in Europe by The Independent, and one of the 10 best by the European Festival Awards.
You can read more on their site. Tickets for 2013 are avaible from november 2012. don't miss it!