http://www.siteinteractive.com/subpro/ is a good script, we've used it for years and had no problems with it

You can also set it up to handle multiple mailing lists from one admin database, which is especially good for paysites as typically programs will have more than 1 site in their portfolio so for example, you can mail your twink site members a different copy than your bear site members copy

Regards,

Lee