The best open source alternative to Infinity is Foam, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 50 alternatives to Infinity and nine of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to Infinity are Notea (Free, Open Source), NocoDB (Free, Open Source), Focalboard (Free, Open Source) and Jira Clone (Free, Open Source).
Foam is a personal knowledge management and sharing system inspired by Roam Research, built on Visual Studio Code and GitHub. You can use Foam for organising your research, keeping re-discoverable notes, writing long-form content and, optionally, publishing it to the web.
Leantime is a project management system for small teams and startups. Designed to help you manage your projects from ideation to delivery. Leantime's Idea Manager allows you to store ideas, communicate them with team members, discuss and move them from idea to validation.
Free: Open source with Apache license. No plans. No limits. You can install it in your own servers. Flexible: You have the complete source code in Java, based in OpenXava framework. So hiding features or adding new ones is extremely fast and easy.