Free memory is a waste of money; you want it all used. Modern operating systems (at least Windows and OSX, I'm not sure about Linux) use otherwise unused memory to cache files from the disk to improve performance and responsiveness.
It may be worthwhile to check activity monitor and make sure you don't have any apps leaking memory (Firefox is notorious for it's memory leaks, so you probably want to close and reopen it ever few days), but having all your memory used is a good thing.