eventup - Find Great Event Venues in the US
Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
EventCast - EventCast is an app that helps you schedule events and easily invite participants to subscribe.
Mobilizon - A free and federated tool to get our events off Facebook
EvenTurbo - It has a solution for any size of organization, be it a small business, local user group, regional training company or a large organization.
DownToMeet - DownToMeet is a platform to organize, find, and attend group events.