I can't believe I beat John Gruber to the news, but Apple's new OS called "Lion" has been out for about half an hour now.
John Siracusa does an excellent job of reviewing the OS for the highly technical folks -- his review came out 3 minutes ago. For the rest, see what's new at Apple.com.
I highly recommend John Gruber's advice on installing.
Be aware that Rosetta's not supported anymore with Lion, so if you're using old PowerPC apps your time is up.