Pc Online Offline Monitoring

hello, I´m living out of the United States, Brazil, and since I work as an IT analyst for more than a decade. Currently pay support for some companies, and I wonder if anyone knows any free or open source software that can monitor computers that are connected (or connected to the web) as this would help monitor very small businesses that are part of my client list. The idea is also to help the media to know when said "we're out of network" would be all or only some (especially servers). Always remember the old ICQ who owned a list of "users" online, and then copied by skype, msn and others. If anyone knows of any software thus would be happy to know, especially if the solution for free. (so recalling that last had the opportunity to use a Kaseya software that made this monitoring trough the internet, despite the detailed level of information and assistance in the diagnosis, this solution ends up bumping the cost of most retail customers of my country.

 

Network monitoring?
Nagios
or Puppet for monitoring.