We are working on prioritizing and filling user requests. We would love to get a better feel for which services have the highest demand for easy integration with Formidable. We have added Zapier integration, and removed all requests that can now be done through them.
Update 5/6/2016: The first of a couple Authorize.net add-ons is ready! This one covers one-time credit card or eCheck payments.
Update 4/26/2016: Stripe integration is now available!
Update 1/10/2016: A WooCommerce integration has been released. A third-party has developed another version as well with all the features you would want for creating products from the front-end.
Update 3/31/2014: Integration with Zapier has been completed. You can now send your entries to Constant Contact, Campaign Monitor, Salesforce, Mad Mimi, Google Drive, Google spreadsheets, Twitter, Facebook, Dropbox, and more.