People have been asking for this since the dawn of the Moodleverse (I joined shortly after the big bang). I have thought about this for my own purposes because I have been experimenting with portable Moodle on a Raspberry Pi (MoodleBox) and I would like to make it very easy for people to go through some activities with the minimum of registration. I run it from a USB battery and so can walk up to a person and say "Hey login to the Moodle that is in my left hand pocket and see what it does"
The closest I can imagine is self registration with stripped back field set and no email authorisation required. I have a feeling this is possible but cannot think of the actual plugins that would do it. So if someone can think of them and post here it might help at least two people.
You might find information on this if you include phrases in your search such as "dangerous" "don't do this because security", as such as set up would allow anyone to get on your system and do whatever bad things bad people do. However for my Moodle about my personage I can live with that.
I just had another idea of having some pre-defined accounts for users to click on with the same password and when someone logs in on that account the link is disabled for a set amount of time. But someone would have to write some code for that.