Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Mentioned By - Learn when your site is mentioned by other sites for free
TechEvents.co - Tech and startup events aggregator
Ultimate Push Notifications by Ecomfit - Retarget customers and protect abandoned cart to boost sale
Facebook Local - Discover places to go and events happening near you
Ethereum Push Notification Service - Protocol for incentivized, blockchain based notifications.