Ketamine
From The Maze Where Realities Converge
Ketamine is a dissociative drug, which means that it works by inhibiting the uptake of N-methyl-D-aspartic acid in the brain. Phenylcyclidine and dextromethorphan are in this class also, but are quite different subjectively. Ketamine is used as an anaesthetic - sometimes for people, but mostly for animals. Take a little bit too much and you'll fall asleep for a few hours, which is completely uninterresting. The trick is to take just slightly less than is needed for anaesthesia.
For an idea of what the experience is like, read A Crazy Sunday, by Conor