ScaleXtreme Basic Edition is completely free, and you can use it to monitor as many of your servers as you like. The product fully supports in-house servers as well as servers running in the public cloud, like Amazon EC2.
Basic Edition offers a number of great features and is simple to install and use.
ScaleXtreme runs in the cloud (is a SaaS product) so there is no central software to download or run locally. There is a small software agent that you install on your server, which is available from the product once you register and login.