VersionOne, an Agile project management tool I like a lot, has introduced a software development kit based in .NET.  They also made a nice SDK Reference Toolkit:

A series of sample applications built on the SDK are provided and can be used out-of-the-box or as learning tools to help expedite the custom application or integration development process. A few examples are listed below.

• Task Manager: A task tray utility that allows users to view, update, change ownership and close tasks, tests, defects and stories within VersionOne, saving time by eliminating the need to log into the application.
• Estimator Tool: Many agile development teams use the practice of relative estimation. The tool supports relative estimation by allowing users to easily divide items or features into any number of groupings and quickly apply the same estimated value to all items in each group, accelerating the planning process while removing unnecessary complexity.
• Screenshot Tool: Helps a user capture a screenshot and directly attach the image to an item within VersionOne, without having to first save to disk.
• Attachment Tool: Permits users to easily attach a file to any asset in VersionOne including stories, defects, tasks, or tests.
• SVN Workitem Tool: Helps users add workitem IDs via drag-and drop into the TortoiseSVN, an easy-to-use interface to subversion, virtually eliminating errors when cross-referencing source code check-ins to the associated task or defect in VersionOne.

Download the SDK from the VersionOne Community Site.