I was recently reading a technical blog from some knowledgeable geek or other and he mentioned that it recommended learning one new language every year. The benefits are two fold - you both learn a new tool, but more importantly, every new language you learn furthers your understanding of the ones you already know. Its the same with spoken languages, if you learn three or four of them you become more fluent in them all as you can see how they all fit together, how nuances are shared between them, etc.