Does anybody know of a good classifieds application written in Java?
Update ... by "classifieds application", I mean an application which handles buying and selling, advertisements etc., something like "http://www.ikiclassifieds.com/" but only written in Java.
Google, as always, would appear to be your friend in this case; "classified ads java" yields various directories which would appear to link to such products, although I can't say I've inspected the results in any great detail.
This is a pretty broad question, soliciting general opinions without providing any kind of real detail relating to what it is you're looking for. You might as well have said, "recommend a blog engine written in Java", for which you could easily have done your own research and found solutions to meet your needs.
More interesting questions might include, "I'm developing a classified ads solution in Java and I'm stuck with this particular aspect", or even, "does anyone have any specific experience with XYZ, the Java-based classified ads system; I'm trying to evaluate it".