Windows 7 BETA was free for almost year before consumer release. The try out period is over for almost a year by now. So, no unless Bill Gates is your legal parent you have to pay for it. Note some universities have programs available for their students to obtain Windows for little or nothing.
There are no legal, free versions of Windows 7 beyond the trial version mentioned by ijack. Short of winning a copy in a contest, you will have to purchase somewhere. If you are a student (with an approved institution and a .edu e-mail address) you can get Win 7 Pro for 29.99 (or so).
As jefe323 and others have said, it is illegal to use a non-genuine copy of Windows 7 (not to mention you have no idea what it will do to your computer). To guarantee genuine Windows, you should only purchase from a reputable retailer, Microsoft Partner, or the Microsoft online store (also the student deal is great if it pertains to you). To learn more about genuine Windows, visit http://windows.microsoft.com/en-US/windows7/genuine