Application pool stopped

Yes, many:

• 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.

You could use Microsoft Debug Diagnostics for IIS.

Debug Diagnostics (DebugDiag) 1.1 is a comprehensive tool designed to help IIS administrators or developers determine why a IIS worker process is crashing, hanging, or memory leak. It offers a simple User Interface to build rules for capturing these common problems with web applications and also offers a built-in analysis system.

There are three components:

• debugging service: collects all kinds of debug information
• debugger host: hosts the Windows Symbolic Debugger Engine (dbgeng.dll) to attach to processes and generate memory dumps.
• user interface: analyze memory dumps, automate the creation of control scripts and show the status of running processes, including services

Here some links to tutorials: Microsoft Support, ASPAlliance.