Apple has officially announced that OS X 10.5 Leopard will be available October 26th for $129 (or $116 with the education discount) and will go on sale in Apple stores at 6PM that night (It seems that Apple is beginning to like the 6PM launch time).
You can pre-order Leopard at the Apple online store here. You can also pre-order it on Amazon.com for $109 here
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Apple has also decided to do a similar program to what Microsoft did, now if you purchase a Mac on or after October 1, 2007 you will be able to upgrade to Leopard for up to $9.95 (for shipping). Click here for more information.
If you are curious about any of the new features that are in Leopard, Apple has made it very easy for you, you can browse all of the new features on one page here.