AWeber This is the newsletter sign up service I use for my "Blogging Tips" newsletter (sign up in the right sidebar of the main page). I love AWeber because you can create follow up emails and set them to go out at different intervals. It's super important to stay in touch with your most loyal followers, and AWeber makes it really easy to do this. My favorite feature is AWeber shows you exactly where each sign up comes from on your site.
Mailchimp Mailchimp does not have a "follow up" tool like AWeber. This is unfortunate because staying in touch with your subscribers is really important. However, I use Mailchimp for my weekly blog newsletter. I send these out manually every week to let readers know the latest blog content. Mailchimp has some great tools for creating an RSS email too. I know that might sound like a bunch of mumbo-jumbo. Check out this post more information on the difference between newsletter emails and RSS emails.