ServerAlert - Free

ServerAlert is an app that monitors your servers/computers and notifies you when one of them stops responding. All you have to do is specify the host name of the computer and the port that you want serveralert to attempt to connect to. No server-side software required. You can also set serveralert to play one of 9 different sounds or vibrate for when that server is down.

The free version is limited to one host. Upgrade to either the Pro or Enterprise versions to get rid of the one host limit.