There is a vast amount of information about Ultimate on the Web. To save you time searching, here's a few of the most useful links.
Wikipedia's Ultimate page
Excellent overview of the sport, including rules and conventions, strategy, glossary, and some good links.
World Flying Disc Federation
World organising body for flying disc sports. Download the official WFDF Rules of Ultimate.
UK Ultimate Association
Organising body for the UK. Info about Ultimate nationwide, including the Tour, National Championships, diary of events, and a database of teams around the country.
Australian Flying Disc Association
Visit the AFDA website for some of the best learning resources around.
UK Ladder League
An informal league system for UK teams. Register your team, find others to play against, set up some games and see how high you can climb the rankings.
Try these searches:
Google - Ultimate Blogs (some by famous names within the sport - great sources of information)