I'm looking for the best tools/techniques for monitoring IIS performance problems and then debugging them. I'm particularly interested in ASP.NET applications.
This may be a combination of tools or there may be something that is good for both?
(I'll add an answer or two based on my experience, for my future reference!, but I hope I'll get some other answers with useful tools to add to my collection)
PerfMon is a good basic tool for monitoring.
There are a number of key performance monitor counters that can be used for keeping an eye a server.
A useful article by Microsoft's Thomas Marquardt suggests the following as the minimum counters that should be used for monitoring an ASP.NET application
One counter that I have found useful that isn't mentioned in that article but is discussed in a Thomas Marquardt blog post on ASP.NET Thread Usage is this one:
Here's a list of "Useful tools and add-ons for IIS troubleshooting and IIS7"