Perl - If you're interested in learning about a more mature language than PHP, which is also used by many web sites. Also very useful for more back end server and data mugging tasks.
Python - Used a lot by some of the 'heavy hitters' such as Google. Many consider it to be 'cleaner' than PHP or Perl and its heavy object oriented approach seems to favor larger app development.
C/C++ - You may or may not use this as much on web dev projects but it comes in
Handy sometimes and looks great on your resume. Also, some people won't consider you a serious programmer unless you know some C. I think there's some truth to that - C will teach you good programming etiquette and after all it’s the basic of PHP and many other languages (including Perl and Python)
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