The "Available" category is the total of the Standby cache, Free memory and the Zeroed list.
Free pages - When a page is taken off of the Standby page list, it is moved to the Free page list. This page is in physical RAM. These pages are not associated with any process. When a process exits, all of its pages are then dumped onto this list. Typically, there is a very small to no amount of free pages hanging around physical RAM.
Standby pages - This is a page that has left the process' working set. This page is in physical RAM. A standby page is like a cache for virtual memory pages. It is still associated with the process, but not in its working set. If the process touches the page, it is quickly faulted back into the working set. That page also has one foot out the door. If another process or cache needs more memory, the process association is broken and it is moved to the free page list. Most of the pages in available memory are actually standby pages.