A drug that inhibits acetylcholine, a neurotransmitter used in the vision, hearing, and memory. This causes vivid hallucinations completely indistinguishable from reality, and usually extremely uncomfortable side effects like restless legs, drowsiness, sensitive eyes (expect to wear sunglasses for days after trying datura), unquenchable thirst, and dry, sticky eyes. The most common hallucinations are small insects, but the hallucinations grow more intense as the dose increases. Users may talk to people who aren't there, see zombies clawing at their windows, smoke phantom cigarettes, see blood dripping from the ceiling, etc.
People take these for fun, but most of them never do it again. The experience is certainly not recreational, but it's like asking people, "if you could live a nightmare in real life, would you?" Some will say yes.
Most deliriants are cheap and fully legal. They include diphenhydramine (Benadryl), dimenhydrinate (Gravol), datura (jimson weed) and amanita muscaria (fly agaric mushroom).
Take it from me, don't try deliriants.