bird - 12:55 pm on Aug 20, 2002 (gmt 0)

On a healthy and active system, there's no "unused" RAM.

Linux (and probably any *nix system) will give each application the memory it requests, and then use the rest for disk caching. Depending on the reporting mechanism, you may indeed see a constant memory useage somewhere between 90 and 100%. If your web server gets more load and requests additional memory later, then the system will sync out some of the cached disk data and reallocate the gained space to the httpd.