The AgileZen C# SDK by Ian Davis allows API access, providing code to communicate with the API. AgileZen offers authentication, formatting, filters, enrichment, and pagination in its developer site. Additional information at http://dev.agilezen.com/
When it comes to design, it can often be quite tricky to get designer and client to agree 100% or even just be on the same page with regards to the direction of the project. Unless communication is streamlined and completely clear from the start, the project can run into a number of disasters, not to mention becoming costly and more time consuming than necessary. Osmosis is a web application that serves as a kind of questionnaire and design brief to help creatives save time at the very start of a project. The Osmosis API is available for developers to integrate this functionality with other applications.
Writing API documentation for software being created with an agile process can be especially challenging because what is being documented is constantly being changed. Project managers and developer leads must assign resources to writers up front, and writers should speak up as soon as issues arise because documentation challenges can snowball much faster in an agile environment.