The Internet Drug Index for prescription drugs and medications
Medications and prescription drug information for consumers and medical health professionals. The most popular drugs and their side effects, interactions, and use.

MedlinePlus:
Drugs & Supplements.

Drugs.com
Drugs.com | Prescription Drug Information, Side Effects & Interactions.

DrugDigest
Drug Digest is an educational, nonbiased drug information site dedicated to helping consumers make informed choices about trade name and generic prescriptions, vitamins, herbs, supplements, and over-the-counter medications.

MedWatch
MedWatch from FDA.