The beta copy of Leopard, the next version of Mac OS X handed to developers last week by Apple, is available on a file-sharing site for downloading.
Apple's legal department tends to frown on things like that. Or, at least they did when the same thing happened with a beta copy of Tiger, the current version of Mac OS X released in April of 2005. Crunchgear spotted the build, code-named "9A466" on a U.K. site called Oink, and it's currently on PirateBay.
Commenters on Crunchgear's site said the build had been pulled from Oink, but it's still available on PirateBay as of this writing. Leopard is scheduled to be legally released in October.