There is no magical plugin that can monitor energy usage from every device type.
Most people write a script that uses SNMP traps to query their PDU's for power usage. You might be able to find one someone already made for your brand of PDU's at nagiosexchange.org , but writing a script (new "plug-in") should be easy. If your PDU's don't respond to snmp requests, you might be able to get the info from your server/device. We use APC brand managed PDU's.
IBM gear is cool because they include sensors on their motherboards that monitor stats like temperature, fan speeds, power usage, etc. IBM also provides good linux support, with apps that can retrieve said sensor information. Your typical home Asus motherboard does this too. IF you can't get info off your PDU's, see if your server boards provide sensor information and what applications are provided by your motherboard manufacturer to access that information.
As for misc devices, i know cisco gear will tell you via SNMP queries how much power it's using. Not sure about other devices, monitoring your SNMP-enabled PDU's is your best bet.
If you can't get power info from your PDU's or servers/devices, your only remaining possibility is to see if your server colocators or whoever is powering your servers will give you some sort of power usage statistics.