Crossplatform Languages that Just Work

Must work on Windows AND Linux.
All major libraries must be crossplatform and Just Work as well.

Name your language.

What language _isn't_ cross-platform? I'd struggle to think of one. Perhaps C#, in the way that certain libraries aren't available in the Mono implementation. Though it isn't a major problem if you're careful.

I suggest using Ada. It's a fine language, shamefully neglected.