http://aamc.org
This is the official web site for the American Association of Medical Colleges. MCAT registration is available at this site.

http://tester.review.com
This is a free practice test done by the Princeton Review. After a few easy registration steps, you can take a practice exam on the verbal, physical sciences, and biological sciences sections.

http://www.review.com
The official Princeton Review web site contains helpful discussion forums and articles.

http://www.kaplan.com
The official Kaplan web site contains helpful discussion forums and articles.