Haha, it's a big holiday in the United States, so don't expect super fast responses this week.

If you are looking for material that closely represents the actual GMAT, then be sure to practice with OG and GMATPrep--these are all real GMAT questions that have been retired.

Kaplan, Princeton Review and Manhattan GMAT are all great resources for practice questions as well. I studied from Kaplan and PR during my own prep, and I thought that they prepped me for the actual test well. I didn't use MGMAT myself, but I hear the questions are very good.

If you are beginning your prep, I recommend taking some time to put together a solid study strategy first. Two resources you should refer to:

Beat The GMAT Study Blog
Beat The GMAT del.icio.us

There are plenty of great links and info in these resources to help you effectively study for the test.

Good luck!