Redmine exposes some of its data through a REST API. This API provides access and basic CRUD operations (create, update, delete) for the resources described below.
Most of the time, the API requires authentication. This is done via HTTP Basic authentication using the regular Redmine accounts. To enable this API-style authentication, check Enable REST API in Administration -> Settings -> Authentication.
At the time of writing, the API is available in both the trunk and the stable-1.0 branch. See CheckingoutRedmine