Though I've knew Oracle has a good learning tutorial named Oracle By Example for a long time, only until few days ago I realized they have a cool page called  Oracle Learning Library which is more fruitful than OBE. This library provides an access to different learning learning information such as, tutorial, OBE and demo etc for different Oracle products and, above all, a good documents searching page. Like!

http://apex.oracle.com/pls/apex/f?p=44785:1:0::NO