C4 on a budget.

Pro-Russian activists in Ukraine have been relying on one particular piece of software to coordinate their activities and share the latest updates in the long-running crisis. It is called Zello, and it turns a mobile phone or a PC into a two-way walkie-talkie.

At the touch of a button, its users can instantly talk to hundreds of fellow activists for free.
What is Zello?

Zello is a free application which turns handheld devices and PCs into push-to-talk two-way radios of the kind normally used by public safety services. "Zello is about voice communication without calls, instant messaging without typing," says the company's website, Zello.com.