Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
PickStackr - Organize and manage your camera roll
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
SlickPix - Instantly create dynamic events from multiple cameras
Meetabit - All the local tech and startup meetups in one place - easy to organize & attend
Carousel - Carousel is the best way to experience Instagram on your Mac.