Pinger is an application to monitor hosts on the local network or in the Internet via ICMP or TCP Connect(). You can specify the frequency down to a 10 second minimum. There is also an option to add a range of hosts (max /24) by IP address. This is demonstrated in the screenshot. The application logs results to a file as well so you can review how long hosts were unavailable. Alerting options include bouncing the dock icon and several sounds that will play when hosts become unavailable.
NOTE: This is not meant to be an enterprise monitoring tool that pings thousands of hosts.
I'm always happy to add features requested by users. Please send me an email if you run into any problems or have any feature requests. My email address is below.
BUG NOTE: The bug Derek referred to below of the spinner after a bit. I have just run into that in testing by adding a bunch of hosts. I'm still trying to narrow that issue down.