3. Donauinselfest

The Buzz -- The biggest open-air festival in Europe! About 3 million people visit the 3-day event every year. The main stage presents the most famous Austrian and International acts, but with about 30 stages spread over 6.5 kilometers of the Danube Island, you'll find something for your musical taste. Entrance is free and the atmosphere fantastic. There's a wide range of food and drink stalls, and, although alcohol is easily available, there is very little trouble, which allows the police to keep a low profile.