Drugs: (mostly alkaloids and related drug compounds)

General Sites:

US-FDA Center for Drug Evaluation and Research:

The Lycaeum: "trip reports" from the users point of view. Information on questionable uses of drug natural drugs.

The Vaults of Erowid – Psychoactive Compounds: Very informative with a pro-drug abuse slant.
Hallucinogenic Plants by Richard Evans Shultes: Classic, beautiful drawings.

Drugs/alkaloids:

Street Drugs:

Addiction Science Network: Featuring the biological basis of addiction

Neuroscience for kids: Drug effects on the brain

National Institute of Drug Abuse:

Office of National Drug Control Policy: White House

Specific Compounds:

Ephedrine:
Ephedra page:

Khat:
Everything about Qat/khat/kat:

Mescaline:
Peyote and other Psychoactive Cacti:

Nicotine:
How Nicotine Works:
The Nicotine Dilemma:
Nicotine and the Brain: