Depends on the side project. C++, Java, and Perl are the languages I am familiar with, in that order. If the problem is the right kind of problem (picking values out of a text file, mainly) then I'll use Perl. Otherwise, C++ or Java, whichever seems like a better fit.

Clojure!

Most stuffs with fewest typings. And so many elegances.

Ruby (on Rails) or Go.

Mostly Python.

Go OR python.