I highly recommend starting off with Cracking the GMAT
, from Princeton Review
. It has the best overall review of the test and very good strategies. It also comes with 4 practice tests on CD, and another one online. The tests are great because they have answer explanations -- be sure to review those, even for the ones you get right.
The other musts are the official materials -- 11th edition Official Guide (it has a TON of practice questions, but no strategy), and the GMATPrep downloadable tests (go to http://www.mba.com
). The GMATPrep tests
Start there, and come here for the Challenges! Lots of good stuff here.