Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Mendeley - Easily organize your papers, read & annotate your PDFs, collaborate in private or open groups, and securely access your research from everywhere.
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Zotero - Zotero is a free, easy-to-use tool to help you collect, organize, cite, and share research.
Calendify - Calendify is an event community where you can share, publish, and subscribe to event calendars. It helps you to stay informed about what you are interested in and what is yet to come.