At least for a while dk-hostmaster, the national registry in Denmark was using http://dk as (one of) their domains. Nowadays they're called "punktum dk" (literally "dot dk") and have dropped their TLD redirect.
I think it's generally frowned upon by ICANN nowadays.
.uz (Uzbekistan) resolves by itself, but the server it points to has an invalid cert so you have to click past an error to see it. Still, it's technically a working naked TLD.
I think it's generally frowned upon by ICANN nowadays.