This is getting a little tedious but every time I see one of these neat utilities get a version bump I am reminded of how useful I have found it at some stage.
Emailchemy is one of them. If you never need it consider yourself lucky, but if you are attempting to shift a number of users off disparate email clients to a new standard then Emailchemy is a huge friend. Since I last used it this utility has only improved. It can read from a huge number of email clients and write the results out in a number of useful formats. It even includes an IMAP server so you can get the mail into a new system easily.
What utilities have you found useful for converting mailboxes and address books?