Brainiac Designer is an open-source visual behaviour tree editor. It allows you to build behaviour trees by using simply drag&drop. The editor supports plugins, exporters and stores behaviours as XML files.
Please notice that the Brainiac Designer is an editor. You have to write your own exporter which generates files for your AI. The editor contains no node logic so it contains no AI. You have to implement the nodes by yourself. Nodes can be added, removed or modified by modifying the source code of the example plugin included.