50 Web Server Monitoring and APM Solutions (For Reference)

You can add http://netsil.com/ to that list. The latest startup on the block in the service monitoring space. Although we (yes, i work for them) do it by using network packets as the "source of truth" without a code embedded agent like rest of the APMs.

A passive packet sniffer and stream processing technology for live, auto-discovered service interaction topology of your application and ability to set up alerts based on SLAs.