C# is available as a free download along with an express version of Visual Studio. This is admittedly only a small step away from Java but it would still extend

C# is available as a free download along with an express version of Visual Studio. This is admittedly only a small step away from Java but it would still extend your language skills and the .NET framework is great for building desktop apps - particularly ones that interact with Web Services - could have been made for the job - actually I think it was. Of course if you are not thinking of Windows then C# again but this time with Mono.