Do practical key-less cryptosystems exist? Hopefully the question isn't too broad or off.

Basically, are there any practical, and by practical I mean secure enough that a ciphertext only attack is infeasible, key-less cryptosystems? For example, there's the Caesar Cipher (I'm aware of the fact that it can pass as a special case of the shift cipher with the key = 3, but let's assume that there is no general case of the hypothetical cipher), however, it's very impractical according to the aforementioned definition of practicality.
Sure, use AES with a fixed key. Since the key is fixed, it could be considered keyless in the traditional sense. Someone with only access to the ciphertext could not crack it as long as you use a good mode (say GCM, CCM, or EAX) using proper nonces, IVs, or whatever else is required.

This is security by obscurity as the plaintext is only as secure as the fixed key. Since the fixed key is on the devices, it is only as secure as the security of the devices. A physical security breach could leak the keys and thus destroy security. A "cyber" attack could do the same.

Note though, that this security will not continue forever. IV's will eventually repeat, counters will eventually repeat, etc.