You do need an account to generate an API key. The API key will work even if the trial it's associated with is ended. No need to pay for an account.
If you are developing an integration that you intend to sell or distribute to multiple users, you should look into our partner program. As part of the program, you'll get a developer account to use while developing your integration as well as revenue sharing for new customers you bring to Constant Contact. I'd be more than happy to talk about how we can help you develop your integrations, feel free to send me a private message with your information and we can set something up. You can also find out more information and apply directly here: http://www.constantcontact.com/partners/technology-platform-partners.jsp
Dave Berard Senior Product Manager, Constant Contact