At the QCon I learned about the site http://www.javablackbelt.com/ . I even scored a poloshirt at their stand. It's a site where you can test your java knowledge in a series of exams. As you pass exams you gain points and get belts. After the first few exams you have to contribute yourself by commenting, editing or supplying new questions. With these contributions you gain points that you can use to 'pay' for the next exam.
Warning 1: These are not easy exams. You need to have a lot of detailed knowledge. Especially for students it might be frightening to see that there are so many corners and special cases in Java they were never told about. This might put you off, but once you get the knack of it, you will learn a lot. You can see it as a free alternative (or good training) for Sun's certification programme.
Warning 2: This site is very addictive! I'm limiting myself to a few attempts per week, otherwise it would take too much time. At this point I have my orange belt en will work towards the green one.