Java

Java is a popular and powerful open source programming language enabling the development of both rich client and web applications.  If you're interested in an extremely powerful and non-proprietary application development language, then you have to consider Java.  While it is a little more intimidating than some other languages it compensates for that with increased productivity via true object orientation.  For web based applications, consider utilizing JSP pages to integrate the power of Java with the portability of browser based applications.