Some websites offering good sample query letters for literary agents include WritersDigest.com, AdWeek.com and WeBook.com. AdWeek provides 23 query letters that were actually accepted, and each one is listed by its unique genre.
We Book has information on query letters, as well as samples and information on the application process, and tips for non-fiction book proposals.
Writer's Digest offers over 60 examples of successful query letters in its series entitled "Successful Queries." It hosts several writer and editor blogs and also provides articles separated by writing genre, goal and level. It has information on competitions, conferences, webinars, workshops and writing software. The site also maintains a community forum.