I'm sure I came across a really nice app that would ping a list of sites or IP addresses every x mins/hours and send alerts when it could not ping.
But I can't find the app back, anyone know of any good ones?
I'm not sure if this sends alerts, but it can monitor up to 250 IP addresses at whatever interval you like. It can display the host, status (as green/yellow/red) and ping time.
On the app store for $1.99: http://itunes.apple.com/us/app/ping/id412751654?mt=12
How about a web service that will do this for you?
Check out host-tracker.com. It will ping an IP address (or http-get or traceroute) for you from a collection of sensor sites. It looks like it will email you error notifications.
I haven't used the site, so can't speak to how well it works.