A lot of Java projects manage their source code via Maven. There is the searchable Central Repository. JBoss has a page of their repositories.
But there are other Maven repositories. Here is a list of a few others that I have used and found via the web, in no particular order.
Note from 2020-12-29: I have no idea if any of these are still working. A lot of them start with “http” and not “https”, so you know they are really out of date.
<repository>
<snapshots><enabled>false</enabled></snapshots>
<id>central</id>
<name>Maven Repository Switchboard</name>
<url>http://repo1.maven.org/maven2</url>
</repository>
<repository>
<id>JBoss</id>
<name>JBoss Repsitory</name>
<layout>default</layout>
<url>http://repository.jboss.org/nexus/content/groups/public-jboss/</url>
</repository>
<repository>
<id>Sonatype</id>
<name>Sonatype Repsitory</name>
<layout>default</layout>
<url>http://repository.sonatype.org/content/repositories/central</url>
</repository>
<repository>
<url>http://download.java.net/maven/2/</url>
<id>hibernate-support</id>
<layout>default</layout>
<name>Repository for library Library[hibernate-support]</name>
</repository>
<repository>
<id>Akka</id>
<name>Akka Maven2 Repository</name>
<url>http://akka.io/repository/</url>
</repository>
<repository>
<id>typesafe</id>
<name>Typesafe Repository</name>
<url>http://repo.typesafe.com/typesafe/releases/</url>
</repository>
<repository>
<id>snapshot-repository.java.net</id>
<name>Java.net Snapshot Repository for Maven</name>
<url>https://maven.java.net/content/repositories/snapshots/</url>
<layout>default</layout>
</repository>
<repository>
<url>http://download.java.net/maven/2/</url>
<id>hibernate-support</id>
<layout>default</layout>
<name>Repository for library Library[hibernate-support]</name>
</repository>
<repository>
<id>springsource-repo</id>
<name>SpringSource Repository</name>
<url>http://repo.springsource.org/release</url>
</repository>
Post updated 2013-08-22_23.07.54
Image from World Digital Library, assumed allowed under Fair Use. Image from the Ashburnham Pentateuch, or Tours Pentateuch, a Latin manuscript of the first five books of the Old Testament from the 6th century or 7th century. Its place of origin is unknown.