Yup, Rangifer beat me to it. Unused ram is wasted ram. All the major operating systems work this way now.
I showed this to a friend yesterday. I have a Macbook with 16gb of ram. 15.99 of it was in use, with 4gb being used as "File Cache". I closed some programs, and the in use memory went down to 12gb, and after a few minutes had creeped back up to 15.99gb.
When I checked the file cache, it grew from 4gb to 8gb. It saw I had ram that wasn't being used, and loaded up the cache with data it thinks I use frequently, so if I need it again, it'll already be in ram.
Don't worry about it, your computer will run much faster this way.