What does WebActivator do?

Solution 1:

According to:

http://haacked.com/archive/2010/05/16/three-hidden-extensibility-gems-in-asp-net-4.aspx

This new attribute allows you to have code run way early in the ASP.NET pipeline as an application starts up. I mean way early, even before Application_Start. This happens to also be before code in your App_code folder (assuming you have any code in there) has been compiled. To use this attribute, create a class library and add this attribute as an assembly level attribute. A common place to add this would be in the AssemblyInfo.cs class within the Properties folder.

Solution 2:

To clarify, it gives you a way of hooking into several application start and application shutdown events WITHOUT having to change any existing code files (previously you had to edit Globals.asax.cs).

This is mostly a big deal when making packages as these events are really useful for bootstrapping Http modules and it is really difficult to write code into existing files.