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 resume execution of the debuggee; 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: polling one or more debug clients for approval to resume execution of the debuggee including informing, with one or more application-level messages, each debug client having not responded to the polling for approval of a number of debug clients having provided an approval to resume execution; and resuming execution of the debuggee only if a number of debug clients providing approval to resume execution of the debuggee exceeds a predefined threshold including commanding the back-end debugger to resume execution; 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.