Hidden features of Oracle Database

As my daily job circles around an Oracle DB I wonder are there any hidden features that could come in handy in day to day work with the DB, both as a developer and as a DBA?

As an Oracle DBA I find that increased knowledge of the visible Oracle database features is far more useful than the undocumented and unpredictable hidden features. If you haven't already, start with the Concepts Guide (pdf) then read the Administrator's Guide (pdf). For further reading options see some stackoverflow questions here and here.

Don't know how "hidden" this is, but since version 9i, the Flashback Query mechanism was introduced.

With the Flashback Query, you can see contents and structure of Oracle objects at a certain point in the past (as long as those informations are still stored in the undo tablespaces), for example:

select * from 
mytable 
AS OF TIMESTAMP TO_TIMESTAMP ('2009-05-16 10:20:20', 'YYYY-MM-DD HH24:MI:SS'));

retrieves the records from "mytable" as they were 5 minutes ago.

Occasionally over the past few years I have been given undocumented parameters by Oracle tech support, but usually the reason they are undocumented (~= hidden) is because they really should only be used for troubleshooting and specific issues.

This page (by Donald Burleson) has a good breakdown on how to find undocumented packages and views, but there isn't much there I've had cause to use...