Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
SocialEngine - Socialengine PHP: Elevating Your Online Community Experience.
Cvent - Cvent's event management software provides event planners with a complete solution to increase event attendance and decrease event costs.
phpFox - phpFox is a powerful social network platform for niche communities
EventMobi - EventMobi is an interactive mobile event app, audience response and engagement platform for conferences and trade shows.
BuddyPress - WordPress social networking in a box.