I bought 3 ZigBee Ploggs and an ethernet gateway so we could plot the energy usage of servers in a rack. Unfortunately the Ploggs are about £100 each as is the ethernet gateway. The Windows SDK to talk to the gateway is in the region of £250. So it's not very cheap at the moment to even have a poke at the issue.
I you buy the binary protocol version of the Ploggs and don't want to use Windows I released some scrappy code on sourceforge called plogger that will interpret some of the protocol.
Web of Things are a group who have looked at Ploggs connected to desktop PCs.
I'm also looking at intelligent PDUs but that's not desktop related...