Have you ever used PhantomReference in any project?

A general diced-up table explanation, from the Java Glossary.

Which of course coincides with the PhantomReference documentation:

Phantom reference objects, which are enqueued after the collector determines that their referents may otherwise be reclaimed. Phantom references are most often used for scheduling pre-mortem cleanup actions in a more flexible way than is possible with the Java finalization mechanism.

And last but not least, all the gory details (this is a good read): Java Reference Objects (or How I Learned to Stop Worrying and Love OutOfMemoryError).

Happy coding. (But to answer the question, I've only ever used WeakReferences.)

I used PhantomReferences in a simplistic, very specialized kind of memory profiler to monitor object creation and destruction. I needed them to keep track of destruction. But the approach is out-dated. (It was written in 2004 targeting J2SE 1.4.) Professional profiling tools are much more powerful and reliable and the newer Java 5 features like JMX or agents and JVMTI can be used for that too.

PhantomReferences (always used together with the Reference queue) are superior to finalize which has some problems and should therefore be avoided. Mainly making objects reachable again. This could be avoided with the finalizer guardian idiom (-> read more in 'Effective Java'). So they are also the new finalize.

Furthermore, PhantomReferences

allow you to determine exactly when an object was removed from memory. They are in fact the only way to determine that. This isn't generally that useful, but might come in handy in certain very specific circumstances like manipulating large images: if you know for sure that an image should be garbage collected, you can wait until it actually is before attempting to load the next image, and therefore make the dreaded OutOfMemoryError less likely. (Quoted from enicholas.)

And as psd wrote first, Roedy Green has a good summary of references.