Snow Leopard 10.6.8 incredibly high memory usage?

This is not all that new. Currently, I have a 13" late 2011 MacBook Pro with 4gb of RAM. Right now, I only have 15mb of RAM free, and for some reason the rest is being used by an unknown process. I'll include a screenshot below, and please help me if you can figure this out. The last thing I want to do is go to Lion.