c'mon people: /usr/bin/purge

Just type in 'purge' at the command line (Terminal.app)

from the man page:

PURGE BSD System Manager's Manual PURGE

NAME
purge -- force disk cache to be purged (flushed and emptied)

SYNOPSIS
purge

DESCRIPTION
Purge can be used to approximate initial boot conditions with a cold disk buffer cache for performance analysis.
It does not affect anonymous memory that has been allocated through malloc, vm_allocate, etc.

SEE ALSO
sync(8), malloc(3)

Alternately, run 'du /' in the terminal for 10 seconds and then kill it with 'killall du'

Rapidly seeking the hard disk tends to free up inactive memory.