I don't have the links off-hand, but there are a number of threads with reviews of different test-prep materials.
Off-hand, I'd say there are several sources that are very important in preparation.
Manhattan GMAT series books, specifically sentence correction and number properties.
With any purchase of a
Manhattan GMAT series book, you get access to 6 free practice CAT GMAT exams - these are very useful to prepare for the exam, and get used to the CAT format (if you havent had one before)
OG11 / OG12, the Official Guide from GMAC. Very useful and has a ton of previous GMAT questions which are no longer being used.
You can download GMATPrep which is free and is also from GMAC. It uses real prior GMAT questions in the form of two CAT GMAT sample exams. Great to let you know where you stand.