One major side-project is C++ and Lua. The other is Python(GAE), Django, JavaScript and Awk.
The only problem is constantly switching Python and JavaScript code over several hours. I can't count how may times I get something as simple as an if statement syntax wrong.
And I'm constantly looking forward to the next opportunity for broadening my horizon, basically looking for a good excuse to learn and apply a new language. But I'm also a person with a distinct thing for languages and their design.
Then again, like somebody else has already stated, learning a new language gets easier every time. I think everybody should learn a few.
Beyond that, I've pretty well stuck with one language in production work for about five years. But that's only because I've mostly done modest-scale web app development, which really only requires one backend language. If you stick to web apps, you can go a long way with a single backend language.
But my current startup is about 50% web app and 50% Linux app. So we're finding more and more need for C.
Also, if you count SQL as a language, then I routinely use SQL (and/or HQL) for data access.
And as somebody pointed out in another reply... if you include Javascript, then yes, definitely.
http://svffap.blogspot.com/2008/09/why-python-for-hardware.h...
Depending on your definition of programming language, I also regularly use LabVIEW to interface with test equipment and MATLAB for simulation and analysis.
It's always good to use the right tool for the job.
I believe that if you know how to program then you can effectively use any language. You will only need to know the little quirks that each language has.
I will repeat what coderdude wrote: It's always good to use the right tool for the job.