vm_stat: virtual memory statistics (free memory)

The vm_stat utility displays the virtual memory usage


Mach Virtual Memory Statistics: (page size of 4096 bytes)
Pages free:                   183268.
Pages active:                 363375.
Pages inactive:               262727.
Pages wired down:             176360.
.....

From this cryptic output the free memory is given by the free pages:
    183268 x 4096 bytes = 715 Mb

Source: http://arstechnica.com/apple/reviews/2001/10/macosx-10-1.ars/8