Check out the website of the AAMC for a free computerized practice test. The MCAT exam #3 is available at no charge, and is in the current computerized testing format.
The AAMC also sells 10 more full length MCATs for $35 each. If you are studying for the MCAT with a group of friends, each person can buy a test and you can pool your resources.
If you live in a large college town with several test prep businesses, call and ask if any MCAT prep programs are offering free practice tests. Many MCAT prep businesses do this to attract customers, and there is no obligation to pay for a course.
If you live a medical school, ask current medical students if they have any MCAT study guides and practice tests. Put an ad on craislist.
Check out the website of MyMCAT, a cooperative wiki-based site where medical school hopefuls can share their MCAT study material.