We are going to assume that you are already familiar with the Google's documented advanced search operators and are familiar with some of the advanced search and filtering techniques (this lifehacker post is a great start). These three undocumented search operators should be excellent additions to the the power-user's quiver.
Why its useful: Filter messages that you have been bcc'd on. If you've been bcc'd on a message, your email address won't appear in the to or bcc headers, so the deliveredto is the only way to filter those messages. For example, we bcc all of our automated outgoing mail to a plus-addressed gmail account and then filter and label those messages using the deliveredto field.
Why its useful: This filter is more reliable than the "list:" filter that gmail has documented as it provides discrete identification for lists that utilize this header.
Why its useful: Some newsletters vary their 'from' email address (they'll use addresses like 'email@example.com') while keeping a consistent reply-to address. The replyto: filter can help filter newsletters or emails that would otherwise be unfilterable.
Have you found other documented features? Tell us what you think below.