Run SQL Server Management Studio, right click on the database and select Script Database as > Create to > file
That's for SQL Server 2005. SQL Server 2000 Enterprise Manager has a similar command. Just right-click on the database > All Tasks > Generate Scripts.
EDIT: In SQL Server 2005, you can select "Database" in the object explorer pane and select several databases in the details pane. Then, right-click on your selection and "Script Database as > Create to > file". This will cause it to put them all into one script and it will include all tables, keys, stored procedures, and constraints.
