I need help, cannot decide!

Hello all,

Right.. here's my problem.
Currently, I have a very good understanding of (X)HTML, CSS, PHP & MySQL. I can also skin Wordpress (not bad, but getting the hang of it), and WHMCS.

Well, I hit my head on the cupboard earlier on, and I'm thinking about learning 1 or 2 new languages.

So here's the question, what language(s) should I learn to top-up my current list of knowledge?

*rubs head furiously*

Thanks.