1. A method for collaborative annotation of a document, the method comprising: generating, by a host computer, a graphical user interface (GUI) that displays a document to annotate; transmitting, by the host computer to a plurality of client computers, the GUI; establishing, by the host computer, a plurality of real time communication modes between the client computers, each communication mode including communication channels between all client computers; receiving, by the host computer from the plurality of client computers, a plurality of annotations entered into the GUIs at the client computers, each annotation identifying a span of words in the document to annotate and including an associated annotation type; updating, by the host computer, the GUI to include for display the received plurality of annotations and inter-rater agreement statistics indicating the level of agreement between annotations received from each client computer and annotations received from the other client computers; and transmitting, by the host computer to the plurality of client computers, the updated GUI.