Nagios is one of the best monitoring tool available in open source. This software has been re-used in a log of monitoring platform available (free or commercial) such as Groundwork or Centreon.
When outages are detected, notifications have to be generated. Classic method are: mail, sms, pager, instant-messenger, rss feed, … Now, there exists a new one: with a R2-D2 video projector! Check out the project web site and the video. Amazing!