Some helpful sites...

International Associations:
http://www.ukultimate.com - National Governing Body for the sport of ultimate in Great Britain and Northern Ireland.
http://www.efdf.org - European Flying Disc Federation.
http://www.wfdf.org - World Flying Disc Federation - governing body for all flying disc sports.

Ultimate Skills:
http://www.ultimatehandbook.com - Coaching and learning website.
https://www.playgroundequipment.com/disc-sports-beginners-guide-to-ultimate-frisbee/ - Beginner's guide website.

Beach Ultimate:
http://www.beachultimate.org - The Beach Ultimate Lovers Association.
http://www.paganello.com - the biggest beach ultimate tournament on European soil.

General Links:
http://www.whatisultimate.com - Lots of information about the game and its history.
http://ffindr.com - Find ultimate frisbee tournaments and teams worldwide.
http://www.pickupultimate.com - Find or list ultimate frisbee pickup games.