System uses too much memory

Current linux kernels uses all available memory for required operations (if build properly). Very often when program terminates, the memory which was used by this program isn't cleaned, it still stored in buffers. Thats the reason why you can see lots of memory usage, even is system doesn't do anything. This isn't bad, because any other process could have same amount of memory (stored data in memory by first process will be overwritten by another process data).