Separate application pools for ASP.net applications in IIS

Separating apps in pools is a good thing to do when there's a reason, and there are a number of good reasons listed above. There are, however, good reasons not to separate apps into different pools, too.

Apps using the same access, .NET version, etc. will run more efficiently in a single pool and be more easily maintained. Most annoyingly, IIS will kill idle app pools, requiring the pool be recreated on each use. If you isolate infrequently used apps you'll impose an unnecessary startup cost on users. Combining these apps into a single pool will make for happier users when they don't pay the startup cost, happier servers when they don't give memory to multiple processes and CPU slices for them, and happier admins when they have to manage fewer app pools.

Reposted from ServerFault, "Why add additional application pools in IIS?"

• AppPools can run as different identities, so you can restrict permissions this way.
• You can assign a different identity to each app pool so that when you run task manager, you know which w3wp.exe is which.
• You can recycle/restart one app pool without affecting the sites that are running in different app pools.
• If you have a website that has a memory leak or generally misbehaves, you can place it in an app pool so it doesn't affect the other web sites
• If you have a website that is very CPU-intensive (like resizing photos, for instance), you can place it in its own app pool and throttle its CPU utilization
• If you have multiple websites that each have their own SQL database, you can use active directory authentication instead of storing usernames/passwords in web.config.