1. Desktop-power management
For every PC and monitor left on at your company 24/7, you're paying between $25 and $75 more per year on power bills. Yes, you may have reasons to keep those machines on, such as being sure they're properly backed up and patched after hours, but by investing in PC power management solutions from companies such as 1E, BigFix, Kace, or Verdiem, you can set policies to ensure that machines automatically power down when they're not in use and get woken up for patching, backup, and so forth.
So let's see: Multiply $50 per year times the number of machines throughout your organization. Compare that to the cost of the licenses for the software. The ROI there shouldn't be too tough to calculate. (For the city of Boston, as an example, it paid for itself in a year.) The environmental benefits are pretty obvious: Less power waste means fewer carbon emissions.
2. Server virtualization
A server virtualization project can be complex. It also might not suit the needs of your organization. Caveats aside, it has the potential to bring significant green benefits to your organization.
Numerous companies have large server rooms or datacenters filled with pricey servers, many of which spend the majority of their days mostly unused yet nonetheless drawing power from the wall and adding heat to the environment.
The idea behind server virtualization is to move the workloads from underutilized servers and consolidate on far fewer machines. I've seen case studies of successful server virtualization projects through which organizations have managed to transfer the work of 1,000 servers down to 270, or 260 machines onto 11.
Considering the costs of not only powering servers but also cooling them, this kind of effort can result in big savings on your power bills. It can also save money on future hardware investments -- and spare the environment the greenhouse gas emissions that would have gone into running and chilling those extra machines.