Frozen Goodies - An advent calendar to get the chance to win daily prizes
Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
25DaysOf - A customizable advent calendar, for everyone
Event Hunt - Startups and Tech Events Curated By Locals
25 Days of Vim - A Vim Advent Calendar
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets