MailMergeApp is an AppleScript Studio application that makes it amazingly simple to create 'merged' mailings using the built in Address Book and Email programs in Mac OS X. The single greatest thing about this application is that it uses the Mac OS X Address Book as your contact list, so if you use address book your mail list is already complete.
All you have to do is type in you message, with spell check and multiple undo's of course, and insert your dynamic tags such as name, phone number, email address, address, and click generate. Then it will send a message to every person in that group, substituting their respective information for tags that you entered.
For example, instead of "Dear Customer" you could enter "Dear <$name$>". The individual email would then read "Dear John Doe". Now your users, friends, or family get a specialized email just for them - no more cold carbon copies.
It's ideal for developers who want to send customized mailings but don't need the full spam friendly power of the expensive merge programs that are available. It has a full featured demo mode that allows you to send up to 10 customized emails at a time. Registered versions can send a an unrestricted number of merged emails per batch.