My Red Hat Enterprise Linux system shows that I am using a lot of memory, even though nothing is running. Why?

Solution Verified - Updated 2015-06-22T19:57:57+00:00 -

Issue

My Red Hat Enterprise Linux system shows that I am using a lot of memory, even though nothing is running. Why?
Is low free memory in Red Hat Enterprise Linux a cause for concern?
My Red Hat Enterprise Linux server is using more than 90% of RAM (as reported by free). Should I be concerned about this?
Our Service monitoring group was reporting false memory utilization alerts, as RHEL5.3 servers could easily reach 99% used memory.
How can we better monitor the memory resources on the servers?
What is a good metric to base memory shortage alerts on?

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