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provided by Gmail or iCloud for incoming email, it's often easier to leave everything online than to archive or delete outdated mail.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).
The name "Lost Photos" is a bit misleading, as it doesn't actually recover photo attachments that have been deleted. Instead, the "lost" refers to photos you may have forgotten about or those that don't exist outside your email. (If you retrieve your email via POP and don't leave it stored on the server, unfortunately there won't be any messages for Lost Photos to scan.)
To use the app, simply launch it and then enter your email address and password. The app will scan your server-side emails and retrieve any photos it finds. You can view all your found photos inside the app and quickly choose to add them to iPhoto, view them in Finder, or share them to Facebook, Twitter, via email (for finding again in the distant future, no doubt). Settings in the app allow you to ignore files smaller than 8k in size so it won't retrieve every logo and icon in an email message. You can also choose to ignore GIF images or images sent before a certain date.
I've used Lost Photos with three different email accounts -- some of which have almost 10 years of emails -- and the app quickly retrieved every single photo I had ever sent. Do note, however, that Lost Photos only works with the following hosted email services: Gmail/Google Apps, Yahoo!, AOL or iCloud/.Mac accounts. Support for additional email services is on the roadmap. As far as privacy goes, Lost Photos does not store your email addresses or passwords.
If you've got an email address with years of image attachments, Lost Photos is a fun and useful app. It's available in the Mac App Store for US$2.99.