Hi Sharmila,
The mock tests usually come free of cost, and I personally feel that they're more than enough. The ones I took are:
Princeton Review (online and book)- in all about 3 in number
Kaplan (CD,book and online)- in all about 7 tests
GMATPrep software- 2 tests
Old Powerprep (though of no practical use, can act as a morale booster!!) they're 2 in number.
Please always do a thorough analysis of the mock tests....Spend atleast 2 hours reviewing the questions -both the worng ones and the correct ones!! I cannot possibly over-emphasize this point!! You may get lower scores in the begining, but don't get demoralised.....Learn from your mistakes.
This is the most appropriate time to start taking your mock tests. Take them all in one sitting. This would help u build your stamina.
Please also check out a success story:
770 (V:44 Q:50) AWA:5
You can PM me if you want more last minute tips and strategies on GMAT. I took mine only last month.
All the best!!
Ashwin