1. A method of collaborative software debugging in a distributed system, the distributed system comprising a debug server, a plurality of debug clients, and a data communications network, the debug server coupled for data communications to the plurality of debug clients through the data communications network, the debug server comprising a debug administrator, a message router, a back-end debugger, and a debuggee, the method comprising: receiving, by the debug server from the debug clients asynchronously during a debug session of the debuggee, a plurality of application-level messages, including receiving, from a requesting debug client, a request to establish a variable-specific message for a particular variable, the variable-specific message comprising an identifier of the requesting debug client, and content to be displayed in association with the particular variable in a debug client's client-specific graphical user interface (‘GUI’) upon evaluation of the particular variable; routing, by the message router in accordance with an application-level message passing protocol, the application-level messages among the debug clients, the debug administrator, and the back-end debugger, thereby providing distributed control of the back-end debugger to the debug clients with application-level messages routed to the back-end debugger, including establishing the variable-specific message for the particular variable; and returning, by the debug server to the debug clients, in response to the application-level messages routed to the back-end debugger, client-specific debug results, including responsive to each request by a variable evaluating debug client to evaluate a variable: determining that a variable-specific message for the variable to be evaluated has been established by a debug client that is not the variable evaluating debug client, and in response to said determining, returning, by the message router to the variable evaluating debug client, the identification of the requesting debug client, and the content of the variable-specific message to display in association with the particular variable along with the evaluation of the particular variable in the variable evaluating debug client's client-specific GUI.