Two solutions come to my mind.

1) Use something like AJAX. An iframe with periodic http requests invoked
by javascripts might might be one solution.

2) Use an applet. This applet can either listen on a specific port for an
event to occur (you can also send even packet from server  bu multicast,
then all ethernet adaptors will catch it, but this is usefull only for
lan, since multicast is closed by default in most routers), or they can go
and ask server themselves, either by an RMI call or anything else.