General Internet sites:

Entire world:
http://festivals.com/

European Festivals Association:
http://www.euro-festival.net/

Baroque music festivals:
http://www.medieval.org/emfaq/concerts/other.htm

United Kingdom:
http://www.artsfestivals.co.uk/a_z/az.html

Germany:
http://www.operone.de

http://www.rondomagazin.de/kalender/kfest.htm

http://www.kulturfeste.de/orte.html

France:
http://www.francefestivals.com/

Hungary:
http://www.artsfestivals.hu/festivals/index.php

Finland:
http://www.festivals.fi/

Belgium:
http://festival.start.be/

Italy:
http://www.worldmusiconline.it/