1. Gmat prep tests from mba.com. They are official and authentic . It has two tests and you can take each test twice ,with insignificant number of repetitions. So , its equivalent to four tests in total. They are official , in the sense they project your score close to real test with an accuracy of +/- 20 points.
2. Manhattan gmat
tests. Buy and Manhattan book like SC Guide and it comes with 6 solid tests. They are the best among all other test prep company offerings. They project your score with an accuracy of +/- 40 points to the official test day score.
3. if you have time also try the free Knewton test.
These are more than adequate but in case you are able to exhaust them all , you can try tests from Kaplan
CD and online free tests.
To gauge your real gmat score based on practice test score use gmat club score calculator available at http://gmatclub.com/gmat-score-calculator/
Edit : Also Gmat Club math tests are meant to increase your quant scores. use them exclusively for quant. I agree to 144144's point of view in total. I just got the access to these tests and yet to try them but yes heard a lot of rave reviews. Above mentioned tests gives you official practice ( gmat prep tests ) and Endurance to sit for 4 hours on test day ( manhattan Gmat
tests ). Gmat club tests
are meant to tweak and tune your quant skills.
Am i worth a Kudo ? Life's around GMAT for the Moment