Notif.me SDK - A Node.js lib to send all your transactional notifications
Party with a Local - Party with a Local is a free app that connects travelers with locals that want to party.
Ethereum Push Notification Service - Protocol for incentivized, blockchain based notifications.
Babble - A more personalized social network
Blockbutler - Real-time transaction notifications for the blockchain
Palaround - Meet people you should be friends with.