Don't you love it when the most obvious solution to a problem is right in front of your face? I recently had to search through my nearly 30,000 Gmail messages to find a message with a specific date, and a general search wasn't returning the result I was looking for.
With a little help from friends on Twitter, I discovered the "Show Search Options" link to the right of the Gmail search bar. Clicking it reveals a host of helpful options, including the ability to search by sender, date, attachment, subject, and more. It's extremely helpful if you (like me) are less than diligent about labeling and filing emails in your inbox — I just used it to find all of the family photos my mom has ever sent.