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.
This is the most potent psychedelic cactus. 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. Some people will go to Native American reservations to try and buy some, this is not recommended.
This is the standard legal cactus. It is the most commonly bought and used on the internet.
This is another common cactus found on the internet. The mescaline content for this cactus is very varied from less than the San Pedro up to twice that of the San Pedro. Generally speaking though this is the more potent cactus and less material needs to be used for the desired effect.