Apple previewed Mac OS X Snow Leopard at the WWDC 08.

Rather than focusing primarily on new features, Snow Leopard will enhance the performance of OS X, and improve quality.
Snow Leopard is optimized for multi-core processors, taps into the computing power of graphic processing units (GPUs), enables larger amounts of RAM and features a new media platform with QuickTime X.
Using media technology pioneered in OS X iPhone, Snow Leopard introduces QuickTime X, which optimizes support for modern audio and video formats resulting in extremely efficient media playback.

Snow Leopard extends the 64-bit technology in Mac OS X to support up to a theoretical of 16TB of RAM.

Another powerful Snow Leopard technology, OpenCL (Open Compute Library), makes it possible for developers to efficiently tap the vast gigaflops of computing power currently locked up in the graphics processing unit (GPU).

Snow Leopard includes out-of-the-box support for Microsoft Exchange 2007 and is scheduled to ship in about a year.
More details on the Apple Snow Leopard page.