Not being required to develop a schema before you build your application is a huge time saver. It enables quick prototyping and lets you mold the structure of your document as you delve into its different uses within your application.
If you need to build complex SQL databases you will find DbDesigner quite a useful tool. It allows you to construct your DB in an intuitive and easy to use environment, where you have a visual representation of the tables and relations contained in your project. You can quickly see the fields in a table or how each table relates to the others. After you are finished, DbDesigner can export the schema of the database into an .sql script, or directly connect to a database backend and build it there.