Here the REST API (a good discussion about REST) will be documented after it is a bit more stable and mature. All URLs to the API start with /api/.
Depending on the Accept-header either JSON (Accept not specified or application/json) or XML (Accept text/xml) can be served.

Resource player statistic summary:
Get statistical summary of all players:
URL: /api/stats/summary
Method: GET
Response: A list of general player statistics for all players. For more detailed stats other resources are neccessary.
Example-Requests:
wget --header=Accept:application/json http:https://localhost:8080/api/stats/summary
wget --header=Accept:text/xml http:https://localhost:8080/api/stats/summary

Resource sheet:
Create and update doppelkopf score sheets which will finally be used for the statistical calculations:
URL: /api/sheet
Method: POST
Params: at least 4 playerIds
Response: A new empty and open sheet with the given parameters will be created. The URL of the new resource is returned in the Location header.
Example-Request: wget --save-headers --header="Content-Type:application/json" --post-data="{\"playerIds\" : [ 1, 3, 4, 8]}" http:https://localhost:8080/api/sheet