CiviCRM - Contact management system for non-profit / activist sector, including donations, memberships, events, SMS, bulk mail & full CRM.
Action Network
Rebel Tools (in development): A toolset built by Activist Handbook as an open source, free, modern and easy to use alternative for small to medium sized organisations. We built these tools for ourselves, and made them available for everyone to help out fellow activists.
GlassFrog: A tool for enacting a structure of holacracy within organisations. See https://www.glassfrog.com/.