You get what you pay for. Free practice questions are good to get a basic idea but you will have to pay for "real" questions in a book, as a CD, or online to get a true sense of what knowledge is needed to pass the exam. The benefit of buying a book is that you can go down to your local bookshop and take some time to look through the questions in the books before you decide which one to buy.
The benefit of buying your sample tests as a CD or online is that they usually come in some form of an exam simulation. That way you can become accustom to answering your questions on the computer.
Use the following free sample tests as a place to start. The more questions you read and answer the better prepared you will be for the exam. Even if some of the sites with free questions are, well, questionable. Here are four good ones:
About.com offers 20 free questions at: http://certification.about.com/od/projectmanagement/a/pmp_test_whiz.htm
Oliver Lehman offers 75 excellent questions at: http://www.oliverlehmann.com/pmp-self-test/75-free-questions.htm
The PMI itself offers a PDF document with 3 official sample questions at: http://www.pmi.org/CareerDevelopment/Pages/~/media/PDF/Certifications/PMP%20Sample%20Questions.ashx
Tutorialspoint offers 200 questions at: http://www.tutorialspoint.com/pmp-exams/pmp_mock_exams.htm
Find the complete list of free sample tests in our Sample Questions at the Resources Section. You can also listen to free samples of our PMP® Podcast.
As you make your way through all the free samples that you find online two things will happen: first you will get used to answering exam questions, which is good; and second you will begin to disagree with some of the suggested answers, which is even better. By disagreeing and then opening up your study materials to review you will learn even more. Therefore, even the worst exam sample question that you can find can teach you how to pass the exam. It's all a matter of your personal attitude and knowing that even a bad question can take you further.
Regards,
Cornelius Fichtner, PMP
Principal Instructor
The Project Management PrepCast™