language agnostic - Pass by reference or pass by value?

When learning a new programming language, one of the possible roadblocks you might encounter is the question whether the language is, by default, pass-by-value or pass-by-reference.

So here is my question to all of you, in your favorite language, how is it actually done? And what are the possible pitfalls?

Your favorite language can, of course, be anything you have ever played with: popular, obscure, esoteric, new, old...