I prefer multiplatform languages, i.e. write-once-run-anywhere. Platform.. don't really care, as the tools I use are also multiplatform.
As for languages, in order of preference:
MSL (mIRC scripting language - my first language that I wrote anything substantial in)
Ruby (Websites, minor scripting, very very versatile)
Bash (Also very versatile, but it's too easy to do things more suited for a "real" programming language)
I also know Batch, but that's becoming less and less useful as time goes on.. I need to start on Powershell.
I know bits and pieces of Java, C++, SQL, and Python.