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It's a bit of an early launch, as Snow Leopard will be available on August 28th; originally, it was due to be out in September. The good news: you can pre-order it right now.
Here's the official description of the latest Mac OS operating system:
"Upgrade from Mac OS X Leopard with Snow Leopard, a simpler, more powerful, and more refined version of Mac OS X. It delivers a wide range of enhancements, next-generation technologies, out-of-the-box support for Microsoft Exchange Server, and new accessibility features. It's the world's most advanced operating system, finely tuned from installation to shutdown."