Gmail API

Gmail has a new API that allows developers to create apps that read or send messages. "The Gmail API gives you flexible, RESTful access to the user's inbox, with a natural interface to Threads, Messages, Labels, Drafts, and History. From the modern language of your choice, your app can use the API to add Gmail features like: read messages from Gmail, send email messages, modify the labels applied to messages and threads, search for specific messages and threads."

It's an alternative to IMAP, but it's more limited than IMAP, so it can't be used to create an email client. It's best suited for apps that send email programmatically, apps that index email, apps that monitor your inbox or any other Gmail label. It was created for services like Boomerang that schedule email to be sent later, mail checker extensions like Checker Plus for Gmail or CRM services like Insightly. The new Gmail API is faster than IMAP when retrieving a thread or performing a search, it provides fine-grained permissions and it lets you query change history to keep mail in sync.