Constant Contact (www.constantcontact.com) is a web based email marketing service. They offer an API which allows you to add people to your Constant Contact account remotely, from your own website.

This extension connects to your member registration form and adds new members to your Constant Contact account when someone submits the registration form. You can pass custom fields across and you can specify one of your Constant Contact Interest Categories to add the registrant to. The extension will optionally notify you of successful and unsuccessful attempts. As well, the status of submissions are logged in your site’s CP log, available in the Admin > Utilities panel.

NOTE: Constant Contact extension 1.1 uses a new Constant Contact API. Please be sure to read the Setup Documentation before upgrading.