What do tech co-ops use for monitoring their servers from the outside?

What software/services are you all using for monitoring your servers from the outside? We have systems for performance and security monitoring but I think we also need something that is from the outside to notify us if the server, application or network have problems.

Ideally it should be hosted on a different infrastructure and network to the things you are monitoring.

This seems to be one of the recommended proprietary SAAS options: