What's the shortest code to cause a stack overflow? [closed]

To commemorate the public launch of Stack Overflow, what's the shortest code to cause a stack overflow? Any language welcome.

ETA: Just to be clear on this question, seeing as I'm an occasional Scheme user: tail-call "recursion" is really iteration, and any solution which can be converted to an iterative solution relatively trivially by a decent compiler won't be counted. :-P

ETA2: I've now selected a “best answer”; see this post for rationale. Thanks to everyone who contributed! :-)

Read this line, and do what it says twice.

All these answers and no Befunge? I'd wager a fair amount it's shortest solution of them all:

1

Not kidding. Try it yourself: http://www.quirkster.com/iano/js/befunge.html

EDIT: I guess I need to explain this one. The 1 operand pushes a 1 onto Befunge's internal stack and the lack of anything else puts it in a loop under the rules of the language.

Using the interpreter provided, you will eventually--and I mean eventually--hit a point where the Javascript array that represents the Befunge stack becomes too large for the browser to reallocate. If you had a simple Befunge interpreter with a smaller and bounded stack--as is the case with most of the languages below--this program would cause a more noticeable overflow faster.

You could also try this in C#.net

throw new StackOverflowException();

Nemerle:

This crashes the compiler with a StackOverflowException:

def o(){[o()]}