Allen White shares his list of interesting perfmon counters:
Paging File(_Total)\% Usage
When Windows runs out of memory it takes large chunks of memory and swaps it out to disk, to the Paging File. Unfortunately, the slowest operation in all computing is writing to disk, regardless of the physical media involved, so swapping memory to disk is naturally going to slow down the performance of your system. Keeping an eye on this counter will help you know when you are encountering memory issues, and you can then take action to resolve the conflicts.
There are dozens of interesting counters you could use, but I appreciate that Allen stuck with 15.