I love tools. And I'm always looking for those that will make my job easier, whatever that job may be. Some of my favorite tools are those that allow you to do "what if?" analysis. Projecting the impact of changes to your infrastructure, be it anything from changing the network routing to adding new servers, is one of those thing where a best guess estimate is important to assessing potential impact.
One of the other advantages of good projection tools is that they display their information in an easy to understand fashion. An ongoing issue for technical people is explaining the impact of changes to IT infrastructure and their impact on business; tools which provide this information in easy to understand graphical formats make it much simpler to get buy-in from non-technical people.
In that vein we get the IT Carbon & Energy Allocation Calculator from APC, an online calculator that allows the user to enter aggregate information, starting with the PUE of their facility, and calculate the cost of operation of the facility in terms of both energy costs and CO2 emissions.
Tools like this are not the ultimate in determining the value and expense of changes to your datacenter but they do provide a quick and easy way to look at simple estimates of what changes in power consumption energy costs, and IT load breakdown can have on your long term energy costs and efficiencies.