One of the biggest markers of quality I've found in the handful of companies I've worked for is in their approach to new languages. Ask every company why they use the language they do. Answers like 'Oh, because we just always have' tend to mean they aren't really interested in the developing world of CS. Which also means they won't appreciate people who want to extend themselves outside of what's already being done.
I've been lucky enough to work at places that have allowed me to investigate solutions to problems using the languages that are most appropriate and that enabled me to learn a lot. During the times I was being stiffled (and I never stayed at those places long) I had no choice but to do stuff in my own time.