"Quick and Free Medication Resources" contains only free resources that are easy to use, free, generally trustworthy, and useful to the average Jo.
Drugs.com - Overall good first site for checking drugs, side-effects, etc.
WebMD Drugs & Medications - Also good.
Drug Interactions Checker - How do your prescription medications interact with each other? With herbs and over-the-counter drugs? With street drugs?
Pill Identification Wizard - You've found a pill. What is it?
A little less quick, but just as free.
Prentice-Hall Drug Guide - Dive deeper into medication use, side effects, cautions, and action. (also a little outdated, 2007)
This list was initially created for The Alder, San Francisco, by the ABC's of nursing students.