At the Auckland and Hamilton Festivals in NZ, there are open mike events named The Homebrew and Drinking Song Workshop.
These are organised by a large and popular man, Lew, who brews the homebrew and dispenses it, and makes sure that no-one gets too drunk. He also makes the rules.
No sitting down. No instruments. Politeness is required. Chorus singing is required from drinkers. He will make new rules as necessary.

The workshop, so called, because it isn't a workshop really, begins after the main concert at about 11pm, and the last 20 hardy souls leave about 4.30, long after the beer has gone, and their voices too. It is held in a small marquee and always well attended, up to 200 people.

In the 7 or 8 years I've been attending, there has never been any trouble or even arguments. There is an official arbiter (who keeps a low profile) for the microphone and Lew and assistants behind the bar until the beer runs out. 4 years ago a microphone was introduced, because the louder voices in the middle were taking most of the songs, and this introduced an element of discipline. Singers queue for the mike, and newcomers are encouraged by the arbiter, who is watching for new enthusiasts. It doesn't really matter if a singer has a good voice, because there is the best chorus you ever heard in your life. Because everyone is standing up, they sing louder, and because they are old hands, it is remarkably hard to find a song they don't know, or quickly learn. They also sing along with the verses!

The high spot of my musical year.