What's the emergency?
Wherever you are, when you hear sirens, launch this app. It will use geolocation to figure out where you are, then check police, fire, 911, and other data to see what emergency activity is going on near you.
It can sort by distance, and also translate 911 jargon into plain English.
what many ideas already posted point to and could use would be a two-way data gathering and dissemination system like NYC's 311. I laugh every time I see an ad for King Co's "Links by Zip" How archaic! We need 311 to digest the data. Then apps to interface with it.
During major emergencies, the cell network breaks under too much voice traffic. SMS requires way less bandwidth. So, why not anticipate the next big outage and work with cell carriers to limit the cell network to sms traffic, thereby increasing capacity by 10x- 100x. This ties in with text to 911 idea someone submitted--which would be an essential component if authorities chose to throttle voice and video traffic.