You can run a headless Linux distro (although that's not exactly turnkey, I know). If you're already running linux, you can hit something like ctrl+alt+F3 to get a commandline only UI: https://en.wikipedia.org/wiki/Getty_(Unix)
Note for anyone who tries this out - to get back to your main session, try ctrl+alt+F1 or ctrl+alt+F2
Note for anyone who tries this out - to get back to your main session, try ctrl+alt+F1 or ctrl+alt+F2