The best open source alternative to Highrise is ONLYOFFICE, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 100 alternatives to Highrise and many of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to Highrise are Odoo (Freemium, Open Source), Dolibarr (Free, Open Source), EspoCRM (Freemium, Open Source) and Mautic (Freemium, Open Source).
Odoo (formerly OpenERP) is a suite of open-source business apps written in Python and distributed under the AGPL license, under the Open Core software business model. It is used by 2 million users worldwide to manage companies of all size.
Mautic is marketing automation software (email, social, & more). Install on your own website or create a free hosted account. The simple 3-step setup process takes less than 5 minutes and you'll be ready to reclaim control of your marketing.
SuiteCRM provides an enterprise-grade Customer Relationship Management (CRM) application to a global community of users and software developers. It is a robust and credible alternative to Salesforce, Microsoft and the major CRM vendors.
Tryton is a three-tiers high-level general purpose application platform under the license GPL-3 written in Python and uses PostgreSQL as database engine. It's the core base of a business solution providing modularity, scalability and security.
Free and open source human resource management software (HRMS / HRIS) with rich features and easy to use interface. It covers personnel information management, employee self service, leave, time & attendance, benefits, and recruitment.