Mailboxer is Smart Mailboxes for your contacts in Mac OS X's Mail.
By default, Mailboxer will create a smart mailbox in Mail for each contact in your Address Book, giving you convenient access to all conversations you had with each of your contacts. If you have a group named 'Mailboxer' in your Address Book, just the contacts in that group will be used. After launching the application you can access a more detailed readme. Be sure to read that.