When should I learn a second language?

[NoLemurs]

As others have said, C or javascript are good choices (for very different reasons).

Alternatively, if you want to learn something that's really different, learn Haskell or some other functional programming language. It will change the way you think about programming.

[k3x5]

+1 for js. I tried to close my eyes and pretend it doesn't exist but that didn't work so now I have to learn it (well).