I have used many different Email Marketing services but at this point I believe the decision comes down to two services; Constant Contact and MailChimp. Both of these services do a great job.
Constant Contact is more mature having more templates and a better html editor. It is more expensive, but if you are new to creating emails and want to have the best tools possible this is the one to go with.
MailChimp is not as robust, but does have a starting free version for less than 500 contacts and 3000 email sends.