If you are using multiple application pools you may want to know which application pool belongs to which process id; for example, if you want to attach a debugger it is handy to know which PID to attach to.
The script iisapp.vbs, located in your System32 on Windows 2003 directory will answer this one. You can run it from the command prompt by typing in the script name, provided you have cscript registered as your default handler for vbs extension files, or agree to register it (you get a prompt following the WSH failure to handle this for you).
Scott Forsyth has the skinny.
I’ve not checked this out on IIS 7.0 on Vista, yet.