Obsidian

Re-Logic announced in a Twitter statement (transcribed in the linked article) condemning the actions of Unity (and in vague terms capitalism's effects on the games industry) that they are donating $100,000 each to open source game engines Godot and FNA, as well as sponsoring both for $1000 a month each.

Here are a couple resources to make moving away from Unity easier:
Godot documentation: Getting started from a Unity experience standpoint
Experimental Unity to Godot exporter