While not free and open source. A number of occupy locations have started using http://cel.ly as a way to broadcast important messages to occupy participants. Direct Action in NYC has an account set up to garner instant communication with movement activists. It might not be the best option, but perhaps it's better than relying solely on Twitter.
Below is the text from http://nycga.net:
An app for android that will immediately alert a selected list of contacts that you are being arrested.
Alert your lawyer, loved ones, etc ... that you are being arrested with a click.
I'm Getting Arrested enables anyone, with one click, to broadcast a custom message to SMS numbers in the event they are arrested.
Very easy to setup and operate. Instructions under help menu.
Inspired by a real Occupy Wall Street incident. Free to the other 99%.
App is very fresh - so please test, provide feedback and check for frequent updates.
Everyone needs wireless, sometimes around town there are wireless hotspots left open at shops, libraries, and other places that you can connect to. Sometimes you may find yourself across the park but out of wireless, or maybe youre someone who has a large plot of land and wants to get wireless at an outbuilding, or to control a robotics project. Here is a solution that can be had with a few basic tools, an hour or two of your time, and is completely mobile.
MF/PL is an organization that redefines the concept of "Internet Hosting Service" in a collective, progressive and collaborative way.
May First/People Link is like a coop: as members, we pay dues, buy equipment and then we all use that equipment as we need to for websites, email, email lists, and just about everything else we do on the Internet.
Share what you want, with whom you want.
Diaspora lets you sort your connections into groups called aspects. Unique to Diaspora, aspects ensure that your photos, stories and jokes are shared only with the people you intend.
You own your pictures, and you shouldn’t have to give that up just to share them. You maintain ownership of everything you share on Diaspora, giving you full control over how it's distributed.
This site is powered by Crabgrass, a software libre web application designed for group and network organizing, and tailored to the needs of the global justice movement. The long term goal is to provide the technical tools to facilitate active, confederal, and directly democratic social change networks.