I've looked into this extensively, and so far, the best thing I've seen is from Lencom.com. It allow scripting and all sorts of nifty stuff. It also allows for additional fields and SQL Server integration.
It is a bit tougher to use than others, but if you have a list where some names are missing, you can script it to display anything you want. e.g. "Dear John" for first name only, "Dear Ms. Smith" when you have a title of courtesy, "Dear ACME representative" when you only have the company name, etc. etc. etc. Overall, it's overkill for most people, but if you want serious power, this looks like the best out there.
A bit more pricy at $99 though... (I've seen a similar one go for $1000.)