<p>I notice that there's frequently an aspnet_client folder under the standard IIS web folder structure. What is this used for? Is it needed?</p>

In the .NET 1.1 days and before, this folder provided ASP.NET with its JavaScript support for the validation controls and other functionality.  If you don't have a .NET 1.1 site  or older running it should be safe to delete it.  I would rename it first to ensure it doesn't cause any problems.