This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
I prepared the exam using Kathy and Bert, and tried all mock exams from the CD.
This site also helps me to understand some difficult topics.
Black Belt Factory is also useful although it is not specialized for OCJP, but you can find many great exams contributed by many developers around the world.