MailSteward comes in three versions - Mailsteward, the MailSteward App (available from the App Store), and MailSteward Pro. At $19.99 the app is the cheapest of the three, but it is not as feature packed as the other two. However, all versions will archive all your POP or IMAP accounts, save attachments, search through your archive for retrieval, and allow you to tag your emails.
Upgrading to the standard or pro versions of MailSteward gives you advanced search capabilities, including the ability to save common searches. They also give you the opportunity to export or merge your databases and schedule automatic archiving. MailSteward Pro can also handle in excess of 100,000 emails, (the other versions don't), but it requires a MySQL database server.
Fully featured trial versions of each tier are available on the MailSteward website, and although they are limited to 3,000 emails, they will give you a great idea of what you can do with this software. They also have some great video tutorials to walk you through the operation of this Mac OS X email archiver app.