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.
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.