Aweber is the one we currently use and it is absolutely top notch. The delivery rate is amazing, the monthly fee is reasonable, and the customer support is out of this world.
Currently, we use Aweber for our free weekly e-newsletter at
Business Owners Online and have nothing to say about them except awesome comments. I am going to try and get the owner of Aweber on one of our teleseminars sometime in the near future.
Aaron