Boxes (GNOME)★★★½☆A simple GUI wrapper around Linux’s built-in virtualization support. It makes simple things easy, but to adjust advanced settings you either need to edit config files manually or use another GUI.
UTM★★★½☆A simple application wrapped around macOS’ built-in virtualization and emulation capabilities. Fewer bells and whistles than the commercial options, but works better for some purposes.
Virt-Manager★★★½☆A front-end manager for Linux’s built-in virtualization/emulation (QEMU and KVM, using libvirt). Much more customizable than Boxes, but missing a few convenience features.