Trello - Infinitely flexible. Incredibly easy to use. Great mobile apps. It's free. Trello keeps track of everything, from the big picture to the minute details.
Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Notion - All-in-one workspace. One tool for your whole team. Write, plan, and get organized.
Event Hunt - Startups and Tech Events Curated By Locals
Slack - A messaging app for teams who see through the Earth!
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets