There's tons of good canyoneering links out there. I've tried to just include the ones I use frequently.
Climb-Utah.com -- One of the best canyoneering guides available. It's the first place I generally check for beta on Utah Canyons. Authored by Shane Burrows, one of the foremost authorities for canyoneering in Utah.
CanyoneeringUSA.com -- This site offers a lot! It has a great Utah canyoneering guide, awesome canyoneering pictures from around the world, canyoneering specific gear and more.
Main site for the American Canyoneering Association.
Canyoneering NorthWest -- The only website devoted to sharing information and ideas for exploring the canyons of the northwest. Good for those wanting to get a different experience from the sand stone canyons of the Southwest.