Gemfile

source "http://rubygems.org"

gem 'tweetstream'
gem 'yajl-ruby'
gem 'growl'

growltweet.rb

require 'rubygems'
require 'yajl'
require 'tweetstream'
require 'growl'

username = 'username'
password = 'password'

twitter_logo = File.join(File.expand_path(File.dirname(__FILE__)), 'twitter.png')
keywords = %w(yankees rays)

TweetStream::Client.new(username, password, :yajl).track(*keywords) do |status|
  Growl.notify do |n|
    n.message = status.text
    n.title = status.user.screen_name
    n.image = twitter_logo
  end if Growl.installed?
end

twitter.png