Xymon is the successor to (rename of) Hobbit which was the successor
to the bbgen toolkit, which had been available as an add-on to Big Brother
since late 2002.

Xymon monitors your hosts, your network services, and anything else
you configure it to do via extensions. Xymon will
periodically generate requests to network services - http, ftp,
smtp and so on - and record if the service is responding as
expected. Through the use of agents installed on the servers,
you can also monitor local disk utilisation, logfiles and processes.

Homepage:
https://sourceforge.net/projects/xymon/
