Since I use all Apple Mac OS X as my primary desktop operating system of choice, I wrote a ruby script ago that mimics the GNU/Linux watch command on OS X a couple of months ago (watch.rb). Little that I knew that the watch package is available within MacPorts.
In short MacPorts is an free and open source package management system built into OS X. (Essentially the BSD UNIX port system). The awesome thing about MacPorts is that it lets you install traditional UNIX userland applications that are not included by default by OS X like wget, and watch.