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

Kommunity - Explore communities that share your passion with millions of people

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

Get Together - Get Together is an open source event manager for local communities.

Bigtent - BigTent is a web-based organization and communication portal for groups.

Openki - Platform where you can search, propose, take part, help organize or publish workshops, lectures...