The SitePoint PHP blog has a tutorial posted giving you a crash course on the Wunderlist API and using Guzzle to connect with it. Wunderlist is a task-tracking, to-do list kind of application with support for multiple platforms and, for developers, a nice and relatively easy to use API.

Wunderlist is one of the most popular task management and list keeping applications out there. It allows you to create lists to which you can then add tasks that are easily checkable and have all sorts of functionality built into them. It has native apps for all the major OS’s and all your data syncs effectively over the cloud. It’s very nice to work with if you are trying to keep yourself organized and you can get a lot done even with the free version.

They've posted the final product in a demo repository but they still walk you through the whole process of setting up a simple application that can get, submit and update data via the API:

• Creating the Wunderlist class (a HTTP client)
• Returning data using the Wunderlist API
• Creating and updating data with the Wunderlist API

They also talk a bit more about the OAuth flow the API uses for authentication and points out that the repository also includes a bit more functionality allowing you to get a list and mark the items as complete with a little Ajax.