Free Server monitoring software?

I'm looking for a free server monitoring software. Basically, I want to know if a server has lost power or is in a down state. I guess what i'm asking for is something that will sent a "PING" every once in a while to check if it's still up and running.

This is in a Windows Domain environment, but I can run the app on Windows or Linux. It really doesn't matter to me.

Maybe, something with a web interface or an app that runs on a single server that I can login to would be fine.

Thanks,