This is a grab bag of techniques that did not seem to fit anywhere else. Many are just parlor tricks intended to give the audience something to look at.
Some left-handed players just play standard guitar. The superior strength and dexterity in the left hand helps with complex fingerings. When right handed playing evolved, the right hand did a lot more finger picking since the flat pick had not been invented. Since much of modern guitar playing uses a pick, the right-hand dexterity is often wasted.
Many left-handed players flip the guitar upside down, or more often buy a guitar meant to be played in this manner. Most players reverse the order of the strings, like Jimi Hendrix. Some do not, however, like classic Albert King.
Pick slide. Scrape the pick along the strings.
Violin bow. Jimmy Page, “Dazed and Confused”
Electric Drill Van Halen (Arguments about who did it first)
Place things under the strings.
Play the strings above nut.
Two guitars at once. Stanley Jordan
Turning left hand over, piano style
Play behind the head. Hendrix
Play with teeth. Hendrix
Spin the guitar around your body. Requires strap locks and wireless pickup. Steve Vai
Duck Walk. Chuck Berry
Smash the guitar. Sounds cool, but only once. Pete Townsend