EMCO Ping Monitor is a powerful tool that will check the availability of systems on your network, or the internet.
To use the program you simply enter the IP addresses or host names of the systems or sites you'd like to monitor (up to five in this free version), click the Start button - and that's it. EMCO Ping Monitor will regularly ping the systems you've specified, as well as displaying basic statistics (monitoring time, uptime, average response time and so on).
If a problem occurs - you lose connection to a particular host, say - then the program will warn you by playing a sound and showing a balloon tip in the system tray. But you can also have it send email alerts, if you like, to yourself or anyone else who might be interested.
And if that's not enough then there are a host of other settings available here, from configuring the ping itself (timeout, packet size, TTL, more) to choosing custom actions that will define exactly what your system does when a particular ping event occurs.
EMCO Ping Monitor Professional takes all these features, and adds more. In particular, it can support monitoring of an unlimited number of hosts, and allows you to configure most settings on a per-host basis. The program is available for $49, and you can read more about its capabilities on the EMCO site.