A variety of naturally-growing cacti contain mescaline, a psychedelic drug with properties similar to LSD and psilocybin. Mescaline-containing cacti are unique legally in that they are often legal to possess for "ornamental" purposes.
Legality and Availability
The legal status of mescaline in the US is ambiguous. Mescaline-containing cacti are legal for ornamental purposes, like poppies, but consumption is illegal.
Similar to LSD and psilocybin mushrooms, mescaline activates the 5-HT2A receptor, which is believed to be responsible for its psychedelic effects. It also activates the 5-HT2C receptor; the relevance of this is not known.