[patio11]

MailChimp. Cheap, wonderfully featured API, pre-existing Ruby code that took my integration time to within 2 hours.

If you're doing iPhone apps at the traditional iPhone price point, though, I don't know how any service which charges for a marginal email is going to be worthwhile for you.

[apsurd]

I use campaign monitor. Maybe its just a developer (rather than designer) thing, but the super-rich design-tastic layout that is mailchimp was actually a huge turnoff for me - I couldn't seem to just find stuff. I really wanted to love mailchimp because the monthly rates make it really really cost effective. But I ended up having to go with campaign monitor even though it is more (if you send a lot) because campaign monitor is just damned elegantly simple. The api docs are simple, the interface is simple, the plans are simple.

Just really could not get over that damned chimp staring at me, talking to me, telling me what to do all the damned time, I guess. =\

[petercooper]

Seconded, MailChimp is mindblowingly awesome. Their feature set is wild and they present everything in a very easy to use way. Their new free plan makes it worth a try too.