Ever go to compile a project and you get an unresolved dependency on some class and you have no idea where it came from? If there was some way to find out what jar the class is in... Well now, there is: www.jarhoo.com. In many cases, you can tell right away what jar you'd need to fix a broken dependency. (Not too many jars have org.jdom classes in them.) But for some cases, especially beginners, figuring out which jar you need to add your classpath can be a daunting, frustrating task.
Jarhoo.com helps find those missing jars. The one downside to jarhoo is sheer number of possible jars. For example, a search for "EntityBean" returns dozens of jars. In practice, however, you'd use something like "javax.ejb.EntityBean" but even that search string returns at least two dozen options. Granted, that still makes it a bit easier at least knowing some options. Another problem is that the site appears to list every version of the jar in its system so you could end up with hits leading to old, outdated jars.
But overall a nice tool to have.