You need to open Activity Monitor and look for all the processes you have opened from software you have installed. You must have a lot of something running to eat up all that memory. What OS X version are you running?
About OS X Memory Management and Usage
Understanding top output in the Terminal
The amount of available RAM for applications is the sum of Free RAM and Inactive RAM. This will change as applications are opened and closed or change from active to inactive status. The Swap figure represents an estimate of the total amount of swap space required for VM if used, but does not necessarily indicate the actual size of the existing swap file. If you are really in need of more RAM that would be indicated by how frequently the system uses VM. If you open the Terminal and run the top command at the prompt you will find information reported on Pageins () and Pageouts (). Pageouts () is the important figure. If the value in the parentheses is 0 (zero) then OS X is not making instantaneous use of VM which means you have adequate physical RAM for the system with the applications you have loaded. If the figure in parentheses is running positive and your hard drive is constantly being used (thrashing) then you need more physical RAM.
Adding RAM only makes it possible to run more programs concurrently. It doesn't speed up the computer nor make games run faster. What it can do is prevent the system from having to use disk-based VM when it runs out of RAM because you are trying to run too many applications concurrently or using applications that are extremely RAM dependent. It will improve the performance of applications that run mostly in RAM or when loading programs.
Download and install EtreCheck 1.9.12. Run it on your system and then post the results here.
If you're running Mavericks, then what you describe is normal. All or almost all memory is usually active. That's what you would want, since if the memory is not being used, it's being wasted.
In Activity Monitor, select the Memory tab and look at the MEMORY PRESSURE graph. If it's in the green zone, you don't have a problem.
No matter what version of OS X you have, applications such as as "Memory Clean" are counterproductive and shouldn't be used.
"All or almost all memory is usually active. That's what you would want, since if the memory is not being used, it's being wasted."
That is nonsense. A system that uses all or most memory is not being efficient. It's being stressed to the point where it starts using virtual memory (disk-based memory.) The system gradually grinds to a halt.
My system has 12 GBs of RAM installed. Rarely will more than 6 GBs of RAM be used. In point of fact Mavericks runs quite properly with 4 GBs or RAM and does not experience memory shortages like yours. To experience a problem such as yours with 8 GBs of memory installed is quite out of the ordinary.