I have listed a few popular choices:

Email Marketing & Surveying Software - iContact

AWeber Autoresponder & Newsletters: Unlimited Follow Up Autoresponders Increase Email Deliverability.

Email Newsletter Software for Web Designers - Campaign Monitor

Email Marketing Solutions from Constant Contact

Email Marketing, Autoresponder, Email Marketing Software - GetResponse

Among all, aweber is probably the most reputable one. But I find the approving process ( on the verification message and importing list) annoying - especially they putting the 'human element' on their review process. CampaignMonitor works wonder for me, but unfortunately too expensive.

Share your experience and discuss on the email marketing / autoresponder tool.