There are various other tools, that can do this, but most of them are shell or perl scripts, spawning many processes, thus much CPU-expensive, especially when one wants continuous monitoring and fast response on target failure.

Alarm Pinger is a single program written in C, so it doesn't need much CPU power even when monitoring many targets with frequent probes.

Alarm Pinger supports both IPv4 and IPv6 and it has been tested on Mac OS X, Linux and FreeBSD.