Resources

The Official Overview of the Exam: A summary of the exam's format and timing from the test writers.
GMAT Wiki: A more concise overview of the exam's format, scoring, and history.
GMAT Registration: Get a sense of where you will take the test and what dates are available. It's good to make a test date far in advance (spots fill up quickly), but beware of exorbitant rescheduling fees.

MBA.com's Preparation Homepage: This is the primary resource for official materials online. The GMATPrep software is a key tool for you to download, though we recommend saving at least one of those tests for when you are finishing your preparation and want an accurate indicator of how you will score.
A Very Good Diagnostic Test: We like some of the Manhattan GMAT materials, particularly their practice exams. We recommend that you take the free test. Take the test timed and follow the prescribed section timeline.
A Good Diagnostic Test: Another good test to try out. Most of the questions are fairly representative of what you'll see on the exam, though the verbal sections are a bit easier than the actual exam.
A Decent Diagnostic Test: This exam from the Princeton Review is only a decent representation of an actual test, but it can be helpful as a practice.

The Official Guide 2018 Bundle: Books + Online 2nd Edition; The latest edition of The Official Guide.
GMAT Critical Reasoning Bible: This book provides a good overview of the basic logic required for the critical reasoning section.
GMAT Sentence Correction Bible: This book provides a good overview of the basic logic required for the sentence correction section.
Kaplan GMAT Verbal Workout: Kaplan is good with reading comprehension. This book offers more or less the same method that Stanley Kaplan developed in his parents' Brooklyn basement during WWII.

I Took The GMAT With No Preparation. Here’s What Happened.: From Bloomberg Businessweek, a journalist takes the GMAT with no idea what awaits her. Spoiler alert: it does not go well.
A Very Charitable Description of What to Expect from MBA.com

Beat the GMAT: Applicants and admissions consultants discuss the exam, though not with any particular degree of trenchancy. Largely a forum for applicants to critique resumes and discuss the relevance of the GMAT to their applications. Sometimes an instructive thread pops up, but not often. Best not to get too wrapped up in this forum.
GMAT Club: Applicants and former test takers comprise this "club" organized around the GMAT. Amidst a heap of corporate advertising, some quality blog posts and practice questions emerge. Again, it's out there if you want to get involved, but we don't recommend it.
Poets & Quants: Sign up for this newsletter to stay in the loop about GMAT and business school news.