Agora is an Ancient Greek word for "gathering place" or "assembly", which exactly represents modern-day conferences and Foss events.
Meetup Mgmt
Conf Mgmt:
OpenConferenceWare is an open source web application for events and conferences. This customizable, general-purpose platform provides proposals, sessions, schedules, tracks and more.
An interesting thread on a Meetup forum: http://www.discussmeetup.com/forum/general-questions-how-tos-tips-tricks/i-want-to-close-my-group-what-are-good-free-alternative-platforms-to-meetup/
For Conferences:
For BarCamps: