Email Archiver is an AppleScript Studio application which sets up 'archive rules' for keeping the size of mailboxes to a reasonable limit. It allows you to specify which mailboxes you want to archive, how old messages must be to archive, and how often to roll over archive mailboxes. It currently works with Mailsmith, a version for Eudora and Apple Mail is underway.
What's new in Email Archiver
New untitled documents no longer appear partially off-screen
Warning when saving mailbox setting with Archive Path that specifies an non-existent mailbox
No longer requires Finder for creating/opening log files
Dialog box when selecting Preferences menu item to inform user that preferences are not currently available