Find unused classes in a Java Eclipse project

I also like to use UCDetector:

UCDetector (Unecessary Code Detector) is a Open Source eclipse PlugIn Tool to find unecessary (dead) public java code. It also tries to make code final, protected or private.

Bonus: it can also find cyclic dependencies between classes

(also a number of other tools -- including Findbugs -- knows how do do that too)

Caveat: Cid mentions in the comments:

UCDetector shall not work if there are interface implementations which will be known only at runtime.
It incorrectly marks the implementation classes as unused.

Update 2017: static code analysis has evolved quite a bit in 8 years.
Using SonarLint for Eclipse, you can use the the latest SonarJava 4.6 plugin to analyze your code.
It will find dead code.

ProGuard can be used to print a report of unused classes/methods. It's a pain to supply all the dependent jars to it, though.

These options list unused classes, fields, and methods in the application mypackage.MyApplication:

-injars      in.jar-libraryjars <java.home>/lib/rt.jar-dontoptimize-dontobfuscate-dontpreverify-printusage-keep public class mypackage.MyApplication {    public static void main(java.lang.String[]);}

Just use Analyze | Inspect Code with appropriate inspection enabled (Unused declaration under Declaration redundancy group).

Using IntelliJ 11 CE you can now "Analyze | Run Inspection by Name ... | Unused declaration"