Apple Events for macOS - The unofficial Apple Events app for macOS
Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Facebook Local - Discover places to go and events happening near you
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
TechEvents.co - Tech and startup events aggregator
Meetabit - All the local tech and startup meetups in one place - easy to organize & attend