Email Marketing Software

I’ve done a little research, but haven’t found anything simple and/or from what I consider to be a reputable “looking” company.

Anybody know of a simple piece of software (free is always better) that will email me when a server goes down? I’d like to enter the IP addresses of about 20 servers, have the software monitor them, then email me if one is unresponsive after a certain duration.

Thanks in advance.
They are my (company’s) servers. Pinging a server is legal and common. Software does exist, free simple software is my goal.