Iranians have been taking to the streets in numbers not seen since the Revolution. Their government is cracking down on protesters both physically and virtually. All cell phone traffic in Tehran has been blocked. The protesters have been using social media sites, especially Twitter, to coordinate and communicate among themselves and with the outside world.
You can help.
If you have an itsy bit of technical skill, go to this guide and turn your computer into a proxy for Iranians to use to access Twitter and other web tools, circumventing government censors. This guide is in windows, it has another for Linux.