SwarmLocal is a free app for iOS and Android that allows you to organize a local event anonymously. The "swarms" you create are centered around your location, so anyone with app nearby will get your instructions of where and when to meet.
This is meant to be an alternative to posting flyers or sending out large group emails. It could be used to plan a flashmob, arrange get-togethers of people with similar interests, or even to just broadcast a message to people in a given radius. Since it's anonymous, SwarmLocal doesn't require any sign-ups, emails, or logins.