Mailing list/newsletter recommendations.

I need some advice on a good mailing list/newsletter plugin for wordpress. I need the ability to give users and opt-in and out option. I then would like to start sending emails to my subscriber list.

Are there any programs that Atahualpa is built to work with already?

Is anyone already doing this with their blog? If so, can you tell me a little about your setup and what program you are using?

Thanks!
Jeff