Snow Leopard offers approximately 32 million refinements both large and small to the Leopard OS, including various uptweaks to the system's response times, a few changes to the user interface, and integrated Exchange support (for those of you whose system admins can be bothered to help you set it up).
Bad news for the three users left still running a G3 system: Snow Leopard is also notable as the first new consumer-level Apple OS release to not feature IBM PowerPC processor support since the processor family's inception in 1992. Snow Leopard will only function out of the box on Apple machines built around Intel processors. Back to the Apple Store with you, gramps!
Windows users are still stuck waiting apprehensively for Windows 7's October 22 release, with an indeterminate amount of time immediately following that for the inevitable service pack releases. With Microsoft no longer offering the Win7 Release Candidate beta for download, I wouldn't be surprised if MS moves its own release date forward as well.