Nimbostratus
Adding a bit to Kevin's excellent response.
The latest version of the SharePoint iApp will allow you to specify custom send and receive strings and select either Basic or NTLM authentication for the monitor. It also lets you choose an existing custom monitor (an EAV, for example).
W/R/T monitors and load balancing, another option is to use F5's WMI monitor together with dynamic ratio load balancing:
http://support.f5.com/kb/en-us/solutions/public/6000/900/sol6914.html
We also have an iCall script that can dynamically adjust pool member ratios based on the self-reported health of the SharePoint servers:
https://devcentral.f5.com/wiki/iCall.Prioritize_Sharepoint_Nodes_on_Reported_Health.ashx
thanks Mike
