Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Bloomerang - Bloomerang is a simple donor database and fundraising software solution that helps nonprofits decrease donor attrition and increase revenue.
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Kindful - Nonprofit donor database + fundraising tools all in one
Bylde - Bylde is a SaaS platform you can use to Start, Manage and Grow your groups and communities.
Classy - Expressive, flexible, and powerful stylesheets for native iOS apps