LearnCPR.org
You can't really learn everything you need to know about CPR on-line, but this site comes close.

CPR illustrations
Or, just teach yourself with some pictures

Learn CPR
The AMA urges you to become a good citizen, with nice graphics. Requires Adobe Acrobat.

ACLS.net
Hints on advanced cardiac life support

Home emergency simulator
How prepared are you for medical emergencies you're likely to have at home? Find out here.

Review of cardiopulmonary resucitation
From the excellent Columbia Home Medical Guide