Shadows is a social bookmarking service for discovering, sharing and managing information on the web. Shadows supercharges this information with a "Shadow Page" — a community blog for any web page that includes views, ratings, tags, and comments by you, your friends and the Shadows community.
For a quick profile of the service see TechCrunch. The API itself has a fairly simple REST-based design with methods for managing posts and tags. As they note it's nearly identical to the del.icio.us API. Authentication is HTTP Basic and developers are advised to follow a small set of 'good citizen' requirements such as at least one second between queries and identifying yourself via your User-Agent string. See this site's Shadows Profile for a summary.