Very extensible CMS platform based on PHP and used by many NGOs - "Community Plumbing"
Open source constitutent relationship management system.
The Social Source Commons is a “knowledge commons” that provides users with: a directory of software applications; links to relevant documentation, localization tools, services, user reviews; and a place to request tools/features.
CiviCRM component: Online signup and membership management.
CiviCRM component: Personalized email blasts and newsletters.
CiviCRM component: Event organising and participants management