Prim to Prim Communication

I am working with having prims communicate with each other regardless of where they are in SL. I am using email with some success. THe sleep period rather sucks but it seems to work pretty well especially if you keep email communcation scripts seperate from the main scripts,
Id like to know if there are alternate forms of communications that you can use to allow prims to "talk" to each other.

If there are other means, do examples exist to experiment with?
Thanks

whisper (10m)
say (30m)
shout (100m)
email (infinite)

As far as *I* know, that's about all there is... other than llGiveInventory

(imagine communicating with an object, by sending it notecard A for certain things, then notecard b for other things.)

You can easily eliminate the sleep period for emails by using a proxy.

llSetRemoteScriptAccessPin and llRemoteLoadScriptPin can be used to communicate integer data to a specific object via its key. Both sender and reciever must have the same owner, and must be in the same sim, so it won't be of any use to the OP. But it's handy for simple stuff like unlinked switches without using an open listener.

If you have access to an external web server, you can use XMLRPC and llHTTPRequest().