pingmond is a daemon program written in Perl which continually pings a target host. It generates logs of high/average/low ping time and packet loss averaged into specified time periods. It can log to text files or to syslog. Everything is adjustable via command line options.
You also might want to see smokeping. smokeping is more advanced and does more, but pingmond can log statistics to flat files or to syslog in flexible formats if you need that.
For more information, see the the man page/pod documentation.