(I'm not against learning a functional language, but that direction has been recommended enough by now)
The advantage of learning something very different is that it will give you a much better overview, and if you ever need or want to learn yet another language, it will be that much easier.
It can help your web apps on the UI side, but it is going to be a major language/player in the next couple of years on the server side due to projects like node.js.
Beyond that c (for when you must go to the bottom of things) and JavaScript (the web language).
Hacking the Python interpreter or making a Python wrapper for a library written in C is a good way to learn C for a Python programmer.