A simple Homebrew extension adding Growl support.
brew install https://raw.github.com/secondplanet/homebrew-growl/master/Formula/brew-growl.rb
Simply follow the instructions on install. If you want to use brew-growl without turning it on all the time,
brew growl install formula
will work. Just use brew growl before pretty much any command. (brew-pip and brew-gem are supported)