Over what kind of distance? How much information? How quickly?
There are lots of answers to your question depending on your answer to those questions.
If you're already having them all hooked up to the same power rails and you can tolerate some noise on them, there are a variety or ready-made ICs which can communicate over power. They'll generally be slower the less complex the supporting circuitry (the homeplug system can do 100Mbps in stuff that's commercially available today, though I don't know what the limitation is on the number of listeners/receivers on a single system is).
If you want to you can just have a buffered and repeated UART signal. I.e. hook the TX line up to a bunch of receivers, and then every 10 devices or so hook up a digital buffer. You'll have to hold the speed pretty low depending on the number of buffers, but it should work well enough.
If you want to go the wireless route there are plenty of cheap and fast options: You can grab a transmitter like this and then as many receivers like this as you want (for the even cheaper version see here). The ones I linked to are kind of a pain to work with, and they're limited in terms of speed (around 4800bps), but they'll do they job.
If you want a more reliable system and you're willing to shell out the money you can just grab any number of branded Zigbee devices. I'm pretty sure Zigbee natively supports sending broadcast messages so this shouldn't be an issue. You'll be a bit limited because every device may respond so they'll be a lot of RF every transmit.
All of this being said: you need to provide specifics if you're looking for specific help!