Psychedelic cacti are one of the ultimate psychedelic experiences. They contain mescaline. Pure mescaline would probably be one of the most popular psychedelics if it wasn't so extremely rare and expensive. The above cacti cause nausea, and are fairly expensive ($20 a dose) for a legal high. You can't just eat the cactus plain either, you need to carefully prepare it and brew it into a drink.
Peyote
This is the most potent psychedelic cacti. It is used religiously by Native American tribes, and can be purchased legally in the USA, if, and only if, you are part of a Native American tribe. Sometimes, other people will go to Native American reservations to try and buy some.
San Pedro
This is the standard legal cacti. It is the most commonly bought and used on the internet.
Peruvian Torch
This is another common cacti found on the internet.