[Don Plumley]

I've used Vertical Response and in general am very happy with it. I like that it is pay by use and not a flat monthly fee. The interface and mail designer is much easier (and also more powerful) than constant contact. My wife's work uses constant contact and trying to make custom HTML was hard.

For very infrequent use you should look at MailChimp. Stupid name, but it's really simple.