Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.

Ethereum Push Notification Service - Protocol for incentivized, blockchain based notifications.

Event Hunt - Startups and Tech Events Curated By Locals

Blockbutler - Real-time transaction notifications for the blockchain

Eventbrite - Discover Great Events or Create Your Own & Sell Tickets

Notif.me SDK - A Node.js lib to send all your transactional notifications