Clearview is what I use.  however there is no "free version" that I know of...at least not when I bought it a couple years ago.  But still for 40 bucks you cant go wrong.  Apples and Oranges comparing it to FMS.

The "free version" only works for free for "X" number of minutes (think its like five minutes)...then it stops working until you buy the key.  I think the developer put this security feature in so you could download and use the FULL flight SIM and all its features to make sure it works with your PC and you actually like it.  But again...once your 5 minutes is up (or however many minutes it is)...it will not work any longer till you put the key in.

Also...you can only install it on once PC.  IF you have a 2nd computer...say a laptop, you can contact the developer and he will send you a 2nd key to use on your laptop.  If you dont...you will have to DE-activate it on the PC...then RE-activate it on the laptop...then back again.

goodluck