The LiveCycle JDBC/SQL component is probably the most useful component in the Foundation category. You can use it to query a database, which can help pre-populate forms, make routing decisions, and more. Its a very powerful component, but it can get a bit complicated, particularly when you want to pass process variables to your query. The blog post below investigates the JDBC component in detail:
http://avokahhh.wordpress.com/2008/08/19/using-the-livecycle-sql-jdbc-component-part-i/
http://avokahhh.wordpress.com/2008/08/19/using-the-livecycle-sql-jdbc-component-part-i/