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.
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.
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.
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.
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.
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.
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.