Java
The Java plugin is used to view Java Applets. These are little applications (applets) that are embedded onto webpages.
Many things on the Web from chat rooms, stock quote trackers and games can require the Java plugin.
Please note that Java is not JavaScript. Many people make this mistake. When someone tells me something is coded in Java, I am 50% sure they mean it. JavaScript is a completely different language and does not require the Java plugin.