The official GMATPrep software is free. You can download it from
https://www.mba.com. I have not downloaded them myself yet, but I think there are two tests.
Powerprep was the previous official software and was also free. But it was created by ETS, who is no longer administering the GMAT. Pearson Vue took over on January 1. You can still find links to Powerprep for 2 additional tests; I think I saw one posted somewhere on the regular GMAT forum. So if you can find that, that's four free tests.
There's another free test on
https://www.review.com, in the GMAT section. You can do the test when you register on the site. You take the test online, rather than download it, but they give you detailed results and group the results by question type. It's cool.