It's often useful to be able to recreate an empty copy of your database. Most popular database systems come with utilities to export your database's DDL to a flat file so you can recreate the structure elsewhere. Here's a list of how to do it on various popular database systems:

IBM DB2

db2look -d databasename [-z schemaname] -e -o outputfile

I've only tested this one on DB2 8.1, but I believe the db2look utility also comes with DB2 7.

PostgreSQL

pg_dump -s dbname > outputfile

MySQL

mysqldump --no-data databasename > outputfile

I've never had to do this in SQL Server or Oracle. If anyone knows the commands for those - or any other popular DB - please post comments!