I love the Unix
watch command. On OSX, you can install it easily with Homebrew:
brew install watch
Something I didn't realize until 10 minutes ago is that if you want to watch the output of something in your bash aliases, watch will complain because it cannot find the command. This is because watch evaluates the command you pass to it with 'sh -c', which does not expand aliases. However, if you also create an alias for watch itself, aliases will work. So, you can add the following to your .bashrc:
alias watch='watch '
Note the trailing space inside the quotation marks.