Aweber
It's been said "the money is in the list", but my friend Craig Perrine states that "the money is in the RELATIONSHIP with the list". And I believe he is correct.
If you have a website and you are not building an email list of visitors, you may be leaving cash and opportunity on the table. There are many ways you can monetize a site by building a list. But it is critical that you use the right service, especially from a deliverability standpoint. After all, if your email doesn't get through to the intended recepient, your efforts are in vein.
That's why I use and endorse Aweber as the email service of choice.