GMAT

BusinessWeek GMAT Prep
Advice on taking the GMAT test and sample practice tests.
http://www.businessweek.com/bschools/gmat/

The Delta Course
Online GMAT study course, test debriefings, and advice on stress. Practice test with immediate feedback. [Requires subscription]. Offers daily GMAT practice questions by e-mail.
http://www.deltacourse.com/

Graduate Management Admission Council
An educational association provides information about MBA programs, financing, student preparation products and GMAT registration.
http://www.gmac.com/

Manhattan Review
Preparation courses in Germany, and MBA admissions consulting.
http://www.review.de/gmat-en.html

The Princeton Review: GMAT
Provides classroom and online courses.
http://www.princetonreview.com/mba/testprep/testprep.asp?TPRPAGE=3

Veritas
Offers preparation and one-on-one tutoring centers.
http://veritasprep.com

Wikipedia: GMAT
Article that gives an overview of the Graduate Management Admission Test.
http://en.wikipedia.org/wiki/Graduate_Management_Admission_Test