PL/SQL Challenege by Steven
This is the link I found few days back. You take part in a daily pl/sql quiz and score. This is a great way of learning and pl/sql lovers will definitely appreciate it. And please note that this is coming from Steven Feuerstein, so has to be good.
This is one shop link to search for the Oracle Documentation. Great thing here is that you can search based on release which is essential with ever changing Oracle features.
You are not learning Oracle if you are not searching this site for answers. Nothing to be mentioned here about "Tom Kyte" as he is one of the highly acclaimed Oracle guru. I personally query this site for any Oracle question/doubt before googling it.