Google Tasks provides a to-do list system for users within GMail, Calendar, iGoogle home page and a direct mobile site. The Google Tasks API provides developers with a set of API endpoints for searching, reading, and updating Google Tasks content and metadata. With the API a user can retrieve, update, delete and create task lists as well as retrieve, update, order, delete and create tasks.
The service is currently in Google Labs, so it's considered to be in beta. The API uses RESTful calls and responses are formatted in JSON.
The ecosystem of “To Do” applications is about to get bigger. Google Tasks, which allows you to manage tasks with Gmail, Calendar and even via mobile web client has just released the Google Tasks API, which could open the door for another set of to-do applications that would be positioned well for people who like to do all their work with various Google services.