I use constantcontact (constantcontact.com) to send out a monthly newsletter, paid version.
It is double opt-in; subscribers sign up, are sent an email to confirm their subscription, and then are subscribed. That way, people can't just add a bunch of emails addresses to a database and then start sending out mass emails.
The other one that comes to mind quickly is IMN (imninc.com).
I came across several others a while ago, but they presently slip my mind. A search should turn up many,