Snow Leopard

Or Mac OS X 10.6.

It's out, it's been reviewed, so I'll add a simple comment.

Perhaps this is Apple's way of compromising with PowerPC users. If you have an older Mac, you are still able to run Leopard (10.5), an awesome operating system with great features and performance. You can't upgrade to Snow Leopard though, because it is Intel only. But, Snow Leopard doesn't include all the new features that differentiated 10.5 from 10.4; in fact, most of it's touted features are aimed at the higher-powered Intel-based machines (Grand Central Dispatch, OpenCL). Therefore, if you are still using a PowerPC Mac, you shouldn't be too put out by not being able to use Snow Leopard, and should stay happy running one of the best operating systems ever, Leopard, on an older machine.

Finally, the last PowerPC-based Macs were made around 2006, so by the time 10.7 or whatever is next rolls around, it will be high time to buy a new machine to take advantage of it.

Image by Ltshears;
and can be found at http://commons.wikimedia.org/wiki/File:Snow_Leopard_Louisville_Zoo.jpg