It's probably not worth learning another language for a web development project. You'll spend most of the time learning the syntax and the framework. Nothing which will expand your mind.

Try a functional language like Haskell, Erlang or ML. But you really need a problem to work on. A database-backed web app can easily be written in Ruby, but maybe there is a feature which requires background processing, write that in the newly learnt language.

BTW, how is your javascript?