Tornado Alert is a community-driven tornado early warning system. Alerts from news sources can take several minutes to appear, and if you lose power you may not have access to the news. With Tornado Alert, any user can submit a tornado warning to other users within 10 miles that there is a tornado close to them. Alerts are normally delivered in under 5 seconds — about 60 — 100 times faster than news organizations tend to report them.
The app is, and will remain completely free, and it only gets better as more people use it. Currently Android-only. iPhone version is pending.
https://play.google.com/store/apps/details?id=com.silverwraith.tornadowatch
http://silverwraith.com/blog/2012/03/tornado-alert-app/