Publr.com is a free blogging service with a few innovative features. It allows posts of various types, including your typical text, but also including maps, videos, chat transcripts, and links. Its site has a pretty good typical web interface for such things, and there are both first-party and third-party applications to use to post. Those third-party applications use the Publr API.
It says this about its API:
The API allows 3rd party applications to interface with Publr. Methods are available for both reading and writing posts. We are constantly adding and improving the API, please check the development blog for updates.
Like most APIs, it is a RESTful service. Responses can be formatted in XML, JSON, or PHP. With it, developers can make posts and get posts. It also uses a formatting compatible to the MetaWeblog API. There are already some good 3rd-party blogging clients that have integrated it, including Pixelpipe.
Blog sites with good APIs are always nice, as it allows a lot more diversity among blog clients, such as clients that allow a posting multiplexed among many different blog sites and other services, such as Facebook. A developer can now make one client that allows posting on an amazing multitude of services, which is really nice for those trying to promote products, among other things.