[kls]

Given your Java experience, you should consider learning JavaScript, jQuery or Dojo Toolkit and JAX-RS. Learn to write JSON services with JAX-RS and to use a client side JavaScript framework to make calls to those services and render the UI. Throw away, JSP, Struts, Tiles and all the other cruft. Use pure CSS, HTML and Javascript to build your UI. With JAX-RS It is as rapid as ROR or Django or any of the other "rapid" frameworks and is drop dead simple.

The reason you want to do this, is because then you know how to build web UI's without any non-W3C technologies. What this allows you to do is take the same front-end and back it with services in any language. So you can start to write JSON services in python, PHP, Ruby or whatever language you want to learn without having to rewrite your UI. This will allow you to try different languages without having to invest heavily in learning large web frameworks. You can write one service in Java and the next in python, the next in Ruby. This will allow you to evaluate which one feels right to you.

After doing this your UI skills are portable across any stack you come across because you have removed the UI from server side stacks.

As for the server side languages for web, PHP is the most popular by a large margin. It is unfortunate given that it is such a poorly designed language, if you are in it for the gigs then learn PHP.

Python would probably be the second most popular (if you exclude Java/JSP and .NET / ASP) and is more rewarding if you are looking for something that has good job prospects with an elegant language.

Ruby would probably come right after the first two, there are jobs and it is a well designed language, worth learning if you want to go that way.