PCP is Phencyclidine. and is related somewhat to Ketamine.
I'm going to add common names or classifications for the drugs on the list. Just to help familarize, etc.
Drugs assessed in order of danger
1 Heroin
2 Cocaine (I'm pretty sure we know what these first two are)
3 Barbiturates (Downers, make you feel drunk, literally. High risk of death in OD. Very widely abused from the 20's-70's roughly)
4 Street methadone (Synthetic opiate used to rehab opiate abusers. I'd guess the risk of the Street variety has to do with purity)
5 Alcohol
6 Ketamine (Special K. Was used as an anaesthetic, but has hallucinogenic properties. Was called a date rape drug, but isn't really)
7 Benzodiazepine (Downers, replaced barbiturates medically because of the death risk)
8 Amphetamines (Speed. Uppers. Used for ADHD, weight loss, and to dance for 12 hours straight)
9 Tobacco
10 Buprenorphine (Opiate used to rehab opiate abusers)
11 Cannabis
12 Solvents (Huff huff)
13 4-MTA (Very similar to Ecstasy with less eupohoric effects, and higher risk of negative effects)
14 LSD (Lysergic acid diethylamide. Acid)
15 Methylphenidate (Upper used to treat ADHD. We know it as Ritalin. Used as a substitute for amphetamines when mixing)
16 Anabolic steroids (See Barry Bonds)
17 GHB (gamma-Hydroxybutyric acid. Also a date rape drug, but again a less common one. Euphoria, sexual arousal, and intoxication at small doses. The doseage curve is very steep though, and negative effects up this curve are far ranging and serious. Very easy to OD on, and it reacts poorly to other downers including alcohol)
18 Ecstasy (MDMA. we've discussed this drug several times)
19 Alkyl nitrates (Poppers. The 'gay' drug of the 60's. Causes a relaxation of the anal sphincter for ease of anal sex. Also causes major drop in blood pressure, and a major increase in bloodflow. Often used during sex to heighten orgasm)
20 Khat (Marijuana of the middle east! Mild Euphoria, generally considered non addictive)
A lot of this is from Wiki, some from my Chemistry book (seriously), and some from random knowledge I've picked up over the years.
I had known what Poppers were, and the blood flow effect (Fear and Loathing in Las Vegas), but didn't know about the widespread use during the 60's and it's relation to gay men.