How to find which classes implement a particular interface in Eclipse?

Try Ctrl+T after selecting the classname; should work in Java Perspective.

You can "Show Type Hierarchy" on the item (Right Click to choose this option or press F4 when the item is highlighted)*.

However, this will list only items in those projects that are referred ("dependent"). Not in others.

So if you have decompiled a jar, and there is another jar that you have not decompiled, then the implementations (of the interface) in that jar will not be listed.

Also, say you have decompiled a jar as a project and there is another project that is referring to the jar, and not the decompiled project, implementations in that will also not be listed.

So you will have to decompile all jars and add them as projects (and add references via "Java Build Path") to make sure all the implementations in the jars of your choice are listed.

_{* Quick type Hierarchy, ctrl + T will show a similar structure in a tooltip/autocomplete kind of panel. But the complete type hierarchy is more useful for the type of analysis you are intending to do.}

Right-click on the interface, and choose "Open type hierarchy". Then click on "Show the subtype hierarchy".

There is radio-button "Implementors" in Java Search.