#1 Hamcrest Core — Never heard of it before. It turns out that this is a library that adds matchers to Junit, making test assertions more readable. Its (for me unexpected) popularity is likely caused by JUnit that depends on it (actually embeds it). The number of inbound dependencies is almost equal (27772 versus 27842 for Hamcrest).
#2 JUnit — is a regression testing framework written by Erich Gamma and Kent Beck. It is used by the developer who implements unit tests in Java. It has more than 10000 direct dependent projects and is likely the most dependent upon project.
#3 JavaBeans(TM) Activation Framework — The JavaBeans(TM) Activation Framework is used by the JavaMail(TM) API to manage MIME data. It is, for me, a perfect example of a library that was over designed during the initial excitement of Java. It has a complete command framework but I doubt it is used anywhere. However, the Javamail library did provide a useful abstraction and it depended on the activation framework.
#4 JavaMail API — The illustrious Java Mail library, developed before there even was a Java Community Process. Provides functionality to mail text from Java (which few people seem to know can also be done with the URL class, but that is another story). Still actively maintained since the artifact was updated less than 10 months ago.
#5 Genesis Configuration :: Logging — Provides the common logging configuration used by the build process, primarily used to collect test output into 'target/test.log'. Surprisingly, it has over 20.000 transitive inbound dependencies. Likely caused by the fact that it looks like every Geronimo project depends on it.
#6 oro — I remember using Oro somewhere south of 1999, it was a regular expression library since Java before 1.4 did not support regular expressions. It turns out that Oro was retired 7 years ago and should not be used anymore. Still it has also over 20.000 dependencies. At first sight, many Apache projects still seem to depend on it even though it recommends that the Java regular expressions should be used.
#7 XML Commons External Components XML APIs — xml-commons provides an Apache-hosted set of DOM, SAX, and JAXP interfaces for use in other xml-based projects. Our hope is that we can standardize on both a common version and packaging scheme for these XML standards interfaces to make the lifes of both developers and users easier. The External Components portion of xml-commons contains interfaces that are defined by external standards organizations. Has not been updated for 7 years (I guess XML's heydays are over by now).
#8 OpenEJB :: Dependencies :: JavaEE API — An open source, modular, configurable and extendable EJB Container System and EJB Server. The popularity of this library is likely caused by the fact that it has an inbound dependency of log4j.
#9 & #10 mockobjects:mockobjects-core — A library to make mock objects. It is over 8 years ago when it was updated but it still has more than 20.000 inbound dependencies.
#11 org.apache.geronimo.specs:geronimo-jms_1.1_spec — Provides a clean room version of the JMS specification. Since this ended so surprisingly high up I look where its popularity came from. It turns out that again here log4j is the culprit.
#12 Apache Log4j — Which brings us to the artifact that is pushing all these previous artifacts to greater heights than they deserve. log4j is directly referenced by a very large number of projects. The following image shows its dependency tree: