Java and Ruby are usually my staple languages for my work. I am working on an AI system for a new customer and I started prototyping everything in Ruby – lots of progress in a short time, but for some things that I want to do Ruby is just too slow so I have separated out the most time consuming calculations and I am writing that module in Common Lisp. Compared to Ruby (and even Java), test cases, etc. in compiled Lisp seem infinitely fast – surprising since for a lot of things like hash table lookups, etc. most of the time is spent in library routines anyway.
I used to have a strong preference for writing entire systems in the same language, but really, it is easy enough to break up systems into modules written in different languages.