Memory Diag is a nice free utility app available on the Mac App Store. Like a lot of memory utilities (including the OS X Activity Monitor), it will analyze your RAM usage, tell you what applications are using the most resources, and offer to clean things up by purging unused processes.
The app lives on your menu bar, and there are three themes to choose from. The menu bar icon is dynamic and gives you a quick look at how much utilization your RAM is getting. I'm often near the limit, which slows up my Mac and forces OS X to use the hard disk as virtual memory. Memory Diag tells you what things are draining your computer's memory. It's possible to have so much RAM filled that Memory Diag can't optimize, but it will suggest which apps to close. Safari can grab a lot of RAM, along with Photoshop (which I am using almost all day long).