As far as I understand, due to license reqirements all web applications, which use MS SQL Server, use SQL Server Express (free) or SQL Server web edition (processor license).
Is it so?
What are other specific features of SQL Server usage for web app?
Essentially. You use a SQL Server Processor license when you don’t want to have to buy Client Access Licenses.
A complete breakdown of the SQL Server Licensing schemes is here:
http://download.microsoft.com/download/1/e/6/1e68f92c-f334-4517-b610-e4dee946ef91/2008%20SQL%20Licensing%20overview%20final.docx