I know this is a pretty old thread but thought I would add Wolfpack to the list for anyone still looking for a "distributed system monitor" - Wolfpack was designed (by me) as exactly that. It can run multiple "agents" collecting data about the servers they monitor and reporting it to a central server instance.
It has a rich set of monitoring and storage plugins and you can easily roll your own for custom checks (there are numerous supporting nuget packages available - just search for Wolfpack on nuget.org) plus there is an active contrib project too...and its really easy to install via chocolatey.org (cinst wolfpack)!
It is open source and completely FREE! I'm also rewriting major parts of it at the moment for the next v3.0 release and it will support SignalR alert notifications, powershell and a full web api/interface (via ServiceStack).
