Android/iOS: If you're a fan of flash mobs, or you just want to organise a get-together of nearby friends quickly, SwarmLocal can help. The app lets you create "swarms" centred on your phone, and then send messages, maps or instructions to anyone in the vicinity.
The video above gives you a hypothetical example of the app in action, but the idea is good for more than just flash mobs and pillow fights. SwarmLocal can be used to broadcast local news to everyone in the vicinity -- something as serious as a traffic alert or as practical as happy-hour specials at the bar down the street. Since the "swarms" you create are centred on your phone, they travel with you in real time. You can set the range (between 1600 metres and 20,000 metres), and communicate with everyone inside the radius at any time.
The kicker of course is that SwarmLocal is new, and it requires other people to have the app installed in order for it to work, so you probably won't fire it up and find a lot going on in your neck of the woods just yet. As the service gets traction though, that will change, and if you're looking for a way to communicate with all of your friends or coworkers in a specific vicinity, it could be a great service to make the job easy.
Best of all, it's free -- no sign ups, no email, no logins. Hit the link below or check out the service's FAQs to learn more.