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.
The team at Aweber has worked incredibly hard to make sure that email from their servers is not blocked by any of the major spam filters. For just $19.95/month, you can create as many mailing lists as you want. This includes the ability to create signup forms, send broadcast messages and deliver mail on your own schedule using autoresponders.
The service actually provides many more features, including RSS feed-based email. If you want to see how this works, visit my blog at JoelComm.com and sign up for my newsletter. You will automatically receive an email each day, nicely formatted and inviting. Once you see what you can do with Aweber, you will be hooked.