Do you know of any that work that way though? I'm just intrigued if any exist. The only example I can think of where a generic suffix resolves separately on its own too is https://gov.uk but that's still a second level domain.

UPDATE: Aha, "to" has an A record. It doesn't respond to an HTTP request for me but it does exist at least.

UPDATE2: OK, there are quite a lot: http://ydal.de/a-records-on-top-level-domains/ .. http://dk/ goes to the .dk registry.