Autoresponder
These autoresponder software and services will help you send sequential emails to stay in touch with your subscribers.

AWeber is a hosted email marketing service that helps you manage and maintain your email marketing campaigns. Features: email newsletters, unlimited autoresponders, RSS to email broadcasts, import existing lists, 150 templates, track clicks, and Twitter and Facebook integration.

GetResponse hosted email marketing service lets you create and send email newsletters, track results, and automate your email marketing campaigns. Features: email newsletters, autoresponders, drag-and-drop editor, 350 HTML templates, one-click segmentation, and analytics.

StreamSend's hosted email marketing service helps you send HTML email newsletters and promotions. Features: 200 to 150,000+ emails per month, real-time reporting, autoresponders, dedicated IP, social share, list importing/exporting, Google Analytics, API, and custom email template design/conversion.

Benchmark Email is a hosted email marketing service to help you build and manage your email lists. Features: autoresponders, 400+ email templates, Facebook integration, iPhone app, RSS-to-email, targeted email marketing, customize online surveys, and list segmentation.

MailChimp is a hosted email marketing service that helps you design, send and track email newsletters. Features: RSS-to-email campaigns, Facebook integration, schedule automated messages, Google Analytics, iPad, iPhone and Android apps, branded forms, and paid newsletter subscriptions.