The very best is the "GMAT Focus" quantitative diagnostic tool available at the official GMAT website MBA.com. This is a fully adaptive test using the same technology as the official test. It is $25 for one test and $65 for 3 tests. I know that free would be better but if you want a quant section that will really test you with official questions - there is no substitute.

You see, the difficulty of official GMAT questions is more subtle than in unofficial questions. An official question that asks something simple like, "is x an even number?," can be extremely difficult. Official questions cost thousands of dollars to write and by the time you see them they have been edited very thoroughly and probably tested on unsuspecting GMAT test takers as well.

This is assuming you have already taken the two free official practice tests also at MBA.com. These tests include verbal as well as quantitative - whereas the GMAT focus mentioned above is just quant.

Good luck!

David