http://www.vmware.com/solutions/home.html
It sounds like you're probably in the market for some kind of consolidation solution -- instead of having 15 servers (each of which is usually running at maybe 10% of its maximum capacity) -- have a handful of big servers and virtualize (so it still
looks like you still have 15 servers, but they share their hardware and you can maintain them all in one place).
You can also do neat things around testing and development -- think you want to upgrade your servers to Vista, but aren't sure if your apps will work right? Set up a new VM running Vista and install and debug everything on it. No need to buy new hardware to test on.
Want to test a product on Win95/98/2K/XP/XP+SP1/XP+SP2? Instead of having a dozen different servers or clients, create images for each and then just load them up as needed as VMs -- you can even emulate networking between VMs. Pretty cool for that kind of environment.