Qubes OS is a Independent-family Linux distribution based on Xen Hypervisor / Fedora & Debian AppVMs. A reasonably secure operating system — compartmentalizing every program into isolated Xen virtual machines.
Strengths: Edward Snowden's operating system of choice; Opening an infected email attachment only compromises that single temporary VM, leaving the host safe; Color-coded window borders show trust level (e.g. Red for untrusted web, Black for password vault). Trade-offs: Demands high CPU and at least 16GB to 32GB RAM; Steep conceptual learning curve.