ITIL (IT Infrastructure Library) is to IT Services what the Project Management Institute's standards are to project management. It has become the de facto standard for IT service delivery worldwide.
Established by the Office of Government Commerce (OGC), the same people that brought us the PRINCE2 methology for project management, ITIL allows organizations to achieve excellence in IT services management. The standard metrics that ITIL provides also allows organizations to benchmark against other organizations.
The savvy IT project manager will want to learn about ITIL, as it makes an excellent compliment to project management skills (be it PMI or PRINCE2).
As an aside, PMI's PMBOK is meant to be a standard, whereas PRINCE2 is a methodology, complete with templates and predefined stage gates.
Check out the OGC's ITIL website...
OGC - IT Infrastructure Library (ITIL)