Mailchimp & Campaign Monitor are the class of the bunch, you can't go wrong with either one. Easy templating, nice stats, A/B testing, autoresponders, API, good

Mailchimp & Campaign Monitor are the class of the bunch, you can't go wrong with either one. Easy templating, nice stats, A/B testing, autoresponders, API, good customer service. iContact and Vertical Response are my next two favs, especially the latter if you send direct mail. I've never used the enterprise solutions, but when I was researching for a client I was gravitating towards ExactTarget. You can do some pretty sophisticated rules for who gets sent what and when based on their behavior.