NSClient++ is a windows service that allows performance metrics to be gathered by Nagios (and possibly other monitoring tools). It is an attempt to create a NSClient and NRPE compatible but yet extendable performance service for windows.
Pretty good mostly, but the license is NOT actually GNU General Public License version 2.0 (GPLv2) as claimed in the project page, when you install it gives you the Common Public License which is similar but incompatible. I don't know why people are dishonest about this sort of stuff, because if you are proud of your code, then choose a license carefully and be proud of your choice. If you are not proud of your choice of license, then rethink why you chose it!