Lost Photos (US$2.99 in the Mac App Store) is a handy Mac utility (OS X 10.6 or later) for finding and rescuing photos that are attached to email messages. In my experience, it works well and works fast.
Despite the growth of Facebook and, more recently, cloud sharing, the most popular way to share digital photos is still email. It's trivial to create a blank message, attach a photo and click "send." In fact, with Mountain Lion's share button, email, Messages and AirDrop share the stage with Twitter and Flickr (with Facebook coming in an autumn update). With the relatively ample storage provided by Gmail or iCloud for incoming email, it's often easier to leave everything online than to archive or delete outdated mail.
While some may consider emailing a photo an old-fashioned approach to image sharing, it offers a nice secondary feature -- a backup of the photo stored on your email service's server. To search for these "backup" photos (and perhaps the only copy of the photo you have anymore) you'd usually have to go through your sent emails one-by-one in your mail or web client, preview the attached photos, and if you want to preserve them, save them to your computer -- a time consuming task.
Lost Photos changes that. The app scans your online email accounts for attached photos. It then moves copies to a single archive location in the Finder which you can treat as you please (add them to iPhoto, edit them in any image editor, etc).