For AKC trials: Enter your dog's AKC info and the site saves it & autofills any trial entry you want. 6-month free trial, $10 a year.
An Agility blog with a lot of nice articles and video tutorials.
One of the first Agility blogs, also has a lot of nice content.
Countryside Agility trial secretaries a lot of the New York and Pennsylvania AKC trials, under their trial information tab you can find trials and their premiums easily.