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

Event Hunt - Startups and Tech Events Curated By Locals

GroupSpaces - Free Group Website, Membership Database, Listserv Mailing List, Online Group Management Software : GroupSpaces

Facebook Local - Discover places to go and events happening near you

The Groups Place - The Groups Place is a new alternative to Meetup.

Eventjoy - The Easiest Way To Organize An Event