I believe it is The Pragmatic Programmer that recommends learning a new language every year. With an emphasis on something that will make you think in a different way.different. If you're in Java, moving to C# is not a big enough switch to make your brain hurt.
It's been 4 years since I read Pragmatic Programmer, but I haven't really put serious effort into any completely different languages. I'm going to make it a goal to start learning Erlang for my LOTY, learning something new each week. Fortunately, I already have Programming Erlang, so I should have some good material to guide me.
If you are in the Java world - there are some great candidate languages that can run on the JVM. Meaning you can start playing with them right now in a Java environment. Scala and Clojure are probably the most frequently mentioned, but I believe there are over 200. This makes it really convenient to try new languages, as if you want to showcase any projects to friends, all they need is Java.
To sum up - there are less excuses to NOT try a new language these days. There are a ton of options out there, so try one and have some fun. I'd love to hear some suggestions.