Trials-online is a good site with an unimpressive home page, but lots of excellent links and information.
http://www.trials-online.com

Monty has been on the forefront of bike trials since the beginning.
http://www.monty.es

Brisa builds an array of stock and mod bikes and components
http://brisabikes.com

Trialsin is a US based site that has bikes, parts, articles and a riders photo forum.
www.trialsin.com

Actually a collection of individual events, the North American Trials Series (NATS) started in 1999 to build the best representation of top-level trials in North America .
http://www.natrials.com

Biketrials.com is a comprehensive site with links, forums and lots of good information.
www.biketrials.com