http://www.heatherbrewer.com/bleedingink/2007/08/how-to-get-published-or-in-response-to.html

Heather Brewer posted her query letter for a book that was published and became a series (midway down the page under the question "What's a query letter?"). The biggest thing to note is how she maintains the same voice in her query as she used for the book. The query is an opportunity pique an agent's interest and show them a little of what they'll see. Voice in the query is important.

http://www.agentquery.com/writer_hq.aspx

This one shows more of the typical structure.

http://www.nelsonagency.com/faq.html#7

This one is advice from an agent on what she likes to see. She includes examples.

http://www.sfwa.org/writing/query.htm

Advice from another agent

http://www.dsmagency.com/published.html

More standard advice.

Hope this helps.