Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Guides by Lonely Planet - Explore city guides curated by experts.
Event Hunt - Startups and Tech Events Curated By Locals
Pretty Streets - Discover cities through customized walking itineraries
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Foursquare 10 - Your ultimate city guide, in your pocket