Unfortunately, you have missed the boat on the Release Candidate download from Microsoft, which let you try 7 for free until March next year.
However, if you were to get a copy of 7 elsewhere, here is a trick which will let you get a solid 120 days evaluation in before you decide to purchase it.
First you need to type regedit in the start menu and navigate to the following key: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\SoftwareProtectionPlatform
Then you need to make sure that SkipRearm is set to 0. Just right click it and choose modify if its not.
The next step is to open the command promt and type the following command: slmgr -rearm
After a restart, you should have 30 more days to evaluate Windows. Remember, you only get three of these, so wait until the last day before you do it. To check how many rearms you have remaining, type slmgr.vbs -dlv in the start menu.