The Instapaper API allows third-party applications to add URLs to Instapaper. Instapaper is a service that lets users save long web pages to read at a later time. The API has a single method. It uses RESTful calls and responses are formatted in JSON.
If you're not familiar with Marco Arment's Instapaper, here's a quick break down. Instapaper is a great little service that allows you to store articles for later on your iPhone or iPad (as of this writing). Instapaper filled a huge gap. Not as high maintenance as an RSS reader, and 100% articles I saw somewhere but didn't have the time to read.
Read-it-later service Instapaper has released what it's calling a "full API." With it, developers using the Instapaper API can create applications that both write to and read from the service. The full API is free to developers, but only paying users can read from their Instapaper account using third-party tools.