
what a pity I saw this thread not 2 weeks ago: there is a HUGE musicfestival every may in Osnabrueck, Germany. it is called Mayweek and is 10 days FREE concerts of ALL kinds on 10 stages.
no, not one stage per day: you have the daily choice of where to go, what to hear on ten (!) stages, PLUS street musicians.
exact dates on Osnabrueck - Germany, search for "Maiwoche"

I can only recommend the Spirit of Woodstock Festival
it takes place near the Lake Maggiore (Northern Italy)
This is a fantastic 10 days festival at the end of July/beginning of August with international bands playing the most various styles (Cyberrock, Dance and Electronica, Metal, Blues, Folk, etc.) They got bavarian beer and Oktoberfest specialities at this festival, yay!!! Don't forget to bring your tent.

summercase festival in barcelona and madrid, on 13th and 14th july. a city festival. air, the chemical brothers, pj harvey, scissor sisters, arcade fire, lcd soulsystem, block party, dj shadow and many more play this year.
primavera sound, also in barcelona. an independent festival, from may 31st to june 2nd. new bands, more than 120 artists. very good one.
these two festivals are in the city, so there's no need to camp. everyone can take the metro to get there, it's very easy and safe. the tickets can be bought online.

Cool camping is totally over-rated. Don't get me wrong, I love festivals and I love camping. But doing it in style? Forget about it! The good thing about festivals is that you can be your scruffy, smelly self. And if everyone is scruffy and smelly then they're even - if just for the weekend. Plus, 'cool camping' tends to apply to those with cars - if you're catching public transport to the event/campsite, you'll struggle to carry anything more than a tent, camp mat, sleeping bag and spare pair of socks.
Anyway, with that whinge over - the Green Man festival in Wales is wonderful. Just the right mix of old wrinkly psychadelic guys and young, laid-back folk fans. A mix of local talent and headliners such as (this year, at least) Robert Plant from Led Zep fame. And best of all, its curated by two musicians called Jo and Danny, a couple who really do care about music and their punters. Beautiful site, too www.thegreenmanfestival.co.uk

Summer music festivals in Switzerland are really good (Paleo, Montreux Jazz) but if you're visiting Switzerland, you should definitely go to Rock Oz'Arènes because it is located in the roman amphitheatre of Avenches. The place is really amazing....Rock'Oz Arenes website

small but GREAT rock & roll fest in belgium
sjock rock 2007
big fest, but loads of good bands
pukkelpop
4 day fest in the french speaking part of belgium.
it's getting kinda big, but has good value for its money.
from techno to punk to metal to hip hop
dour festival

Portugal’s Festival of the Mediterranean 2007
Annual world music festival dedicated to the Mediterranean, will take place June 27 - July 1, in the historical center of Loulé (Algarve), Portugal.
The lineup this year includes Aynur (Turkey); Sara Tavares (Portugal); Sergent García (France); Natacha Atlas (Egypt); Tinariwen (Mali); Akli D (Algeria); Yerba Buena (USA); and others.
Go and enjoy!
http://www.festivalmed.com.pt/</a>

yes, Rock'Oz Arenes (Avenches, Switzerland) is definitly a must!!! There are some more open-air festivals in Switzerland, which are mostly worth a visit: Gurten Open-Air near Bern, Gampel Open-Air which is in the middle of the alpes (Wallis)...

For classical music lovers there is an outstanding festival at Verbier in Switzerland. This summer, for example, the likes of James Levine, Renee Fleming, Evgeny Kissin, Lang Lang, Rene Pape, etc. will be there. There is also an opportunity for amateurs to do some chamber music the week before the festival starts.