Ushahidi is an open source real time crowdsourcing tool that maps crisis incidents sent in through mobile phones. Like wordpress or tumblr, Ushadhidi is a platform. You can embed a map into your website or create your own separate version.
It has been used by activists to track conflict in places where reporting is difficult or goes unreported. Tweets and SMS text messages can be used to send information to the Ushadhidi site. It plots them on an interactive map where users can tag them by category or issue. Other users and hosts can then verify the reports. Crowdmapping is a more user friendly platform created by same people, whereas Ushadhidi can be customized by more tech savy users and programers.
Ushahidi means testify in Swahili. It was originally developed to track the fallout after the 2008 election in Kenya. More recently, it's being used to track eyewitness accounts of violence on the ground in Syria. It continues to evolve and be used by activists all over the world.
Though meant to track disasters and crises, activists can easily adapt the tool to track incidents of violence during a large protest and use it as an alternative new source. Think about the G20: what if the activists had a map of Toronto, where other protesters could see where police were kettling or detaining people. It helps people get a better idea of what's going on as its happening.
Aljazeera was the only mainstream news organization that has used Ushahidi. They embedded it on their website to track violence in the Gaza strip. Ushahidi has mobile apps for the iphone, ipad and android. They use your phone's GPS to give exact co-ordinates.