Threads share the same address space so the easiest way to communicate between threads is altering common objects. Just remember to wrap the accesses to the objects with mutexes, semaphores, critical sections, or whatever else is apropriate.

ah, well if you had the foresight to use Java (servlets or even clients (applets)) :p , you could make a persistent serialized object which can be parsed or even picked up by (any number of) threads (local or remote), and even (if your brave) point the object @ a database (kinda like Apache::Session (perl/CPAN)) for storage/security (which is nice ).