If I'm doing some front-end stuff, I'll use Javascript. I most certainly will not use something like GWT because I want my entire codebase to be Java-only.

I've used Ruby, Python, and others. I'd like to get back into Haskell at some point when I find the time. My current little side project is written using (mostly) Go and Javascript (I'll have to post a "Show HN" sometime, maybe).

Right now I'm really digging Go for network services :)