What kind of licensing do I need for SQL Server ?

One of the first questions at the startup of a project is, what kind of licensing do I need for the SQL Server Stack. And How do I have to count the numbers of licenses. Well actually Microsoft have published a real good document about the different kind of licensing models and how to calculate the number of licenses even in a virtual environment. Go to http://www.microsoft.com/sqlserver/2008/en/us/licensing.aspx for the SQL2008R2 quick reference quide.