Roskilde Festival is North Europe's biggest festival which has been rocking out since 1971. What's more, the festival is a non-profit project celebrated by close to 100,000 happy festival-goers. They're mainly from Scandinavia, but the vibrance of this festival attracts people from all corners of the globe. There are four "warm up" days before the four days of music actually starts, so that everyone can hang out and uncover adventures of all kinds (like participate in the traditional Naked Race!!).
I don't think I've ever experienced such freedom and friendliness - everyone simply leaves their troubles and inhibitions in the city, for a huge party out at an oversized country paddock.