Checklist.com is a web service for the simple task of managing checklists. It seems like a simple task, but it does it surprisingly well, including allowing access to pre-made checklists for a lot of common tasks. It recently opened its Checklist API to the public, and that API allows full access to its services for both commercial and non-commercial use.
Here's how the company explains its own API:
Checklist.com provides a checklist platform that can be used by anyone to manage their precious time and Get Things Done! We provide additional access to this platform via a simple RESTful API. Applications can query the powerful, yet simple, API to get checklist templates or specific user checklists, tasks, contacts & settings. Applications can also make updates to this information. In essence, the full functionality of the platform can be accessed via the API.
It uses a RESTful service, with responses formatted in simple XML. Using the API, one can make new checklists, update existing ones, utilize freely available templates in making new lists, and manage contacts, allowing a user to share checklists with other users. User account logins are handled through the common OAuth standard.
It allows searching for checklist templates via tags as well. A programmer could use this API for a lot of interesting things, especially after creating a few templates. For example, they could make checklists for such tasks as doing taxes, moving, doing a resume, or anything else they could think of, and integrate them into their web-connected application.
To use its API, contact Checklist.com through its form for an API key. They will give you a key that allows you to use their sandbox first. When you are done with your application, you'll need to contact them again to get a key for the live production server.