Java is mainly used on the server to server data to web applications. Applets (small Java apps embedded in the browser) are all but dead (people didn't like the 5-60s wait until the app started). Java never made a big impression on the desktop because deploying Java apps is so unnecessarily hard. After 15 years, there still isn't a built-in, one-click "turn this into a desktop app" which downloads the runtime and everything else it needs on demand.

Java is most heavily used in client-server business applications, either through a web application interface (browser access) or a web service interface (SOAP/REST services, or older style Java RMI).

The Java libraries for developing these types of apps are very mature, and are often considered among the best selection for any language. The Apache Struts ^[1] library, released almost 10 years ago, was a big factor in promoting the development of MVC pattern web applications (and using Java to do it). Microsoft didn't come out with an "official" MVC framework for ASP.NET ^[2] until 2008.

As far as application servers go, there are many choices ^[3] with Java, which all meet the defined server specifications. This is also a factor in Java adoption - you can choose a completely free open-source server, a free server with a support package purchased from an external company, or a full commercial server. With .NET, you just have the official .NET server framework to choose from. Makes the choice easier I suppose, but removes your options (including any free ones).

All over the place. Obviously, for the server-side of client-server programming - web applications, enterprise applications, database backed stuff etc.

It's used heavily for mobile development - lots of not-smart phones have a Java Mobile Edition (JavaME - formerly known as J2ME) VM on them. Android doesn't use a JVM, but has a virtual machine called Dalvik which uses the Java syntax and is compatible with large chunks of the Java platform. There are also the Java smartcards which Sun was hyping a while back, and they are now hyping up SunSpots which are basically Java-based "smart tags" - sort of a bit like an Arduino and an RFID tag but which you can code for in Java. (Admittedly, the success of the iPhone has led people to Objective C and XCode over JavaME.)

There's a few cross-platform GUI apps. Vuze (formerly Azureus), jEdit, Limewire, Jave ^[1] (a nice ASCII art editor), FreeMind, BlogBridge etc. Sun have a page titled Swing Sightings ^[2] which lists all sorts of GUI apps. There's probably plenty more GUIs put together behind the corporate firewall in Java.

Personally, I use it for doing high-performance batch processing - basically, anything that will take a bit too long to run in Python or Ruby, I tend to do in Java (or Scala). Lots of people would do that in C/C++, but I tend to use Java because I'm familiar with the platform.

The strength of Java is also a weakness: it's a jack of all trades, but not necessarily a master of any.