ExamLab released a new mock exam simulator for SCJP / OCPJP exams, for free. ExamLab is now a web-based exam simulator that gives the same look and feel as of the real exam, with carefully made serious questions. It contains,
1 Diagnostic Exam, 4 practice exams, and 1 final exam - all are for free.
Start practicing: http://www.examlab.org