Spaces is Apple's implementation of multiple desktops. If you've used UNIX systems before you're already familiar with the basic functionality (I've even used it on Gir via a 3rd party utility). Spaces has the added niceness of using an Expose'-like interface to view all of the desktops simultaneously, switch between them, and even drag items between desktops. You can also peg apps to open in a specific desktop, and clicking on an app's Dock icon will automatically jump you to that desktop. Two things I'm curious about are how it'll handle clicking the Dock icon for an app that has windows active in more than one desktop, and how it'll handle apps that run full-screen (DVD Player and games). I really hope that running, for example, WoW in full-screen mode won't inhibit access to other desktops. We'll see.
Go here and listen. It's a demo of Alex, a new voice that's optimized for ease of comprehension at high speaking rates. Vision-impaired people who use screen readers usually run at a very high rate of speech, so this is ideal for them, but there's another little touch that Alex has that shows Apple's attention to detail. Listen to the demo again, closely. Did you catch it? It's subtle. Alex doesn't just sound clear and natural. He inhales between sentences. It's almost creepy. Interestingly, this makes him easier to understand. I'm still going to stick with Zarvox for Gir's voice, however.
iChat has a bunch of new bells & whistles as well, including one feature that is almost certainly going to result in me buying it for both our own Macs and also my mom's. It's has VNC integrated with it, so you can remotely control another machine, and do voice chat simultaneously. I've got VNC installed on my mom's iMac, but since her ISP uses NAT I can't actually connect to it from outside (she gets a 192.168 IP address). With the new iChat that won't be a problem. It even automatically switches to voice chat when you activate remote control.
iChat also allows you to share photo album presentations from iPhoto, PowerPoint presentations, etc., and has new photo booth special effects for video conferencing.
Time Machine is idiot-proofing at its best. You wouldn't think that something as nuts & bolts as version archiving (which has existed at least as far back as VAX/VMS) would be novel. Versioning isn't just regular backups. It's the idea that if you make a bad change to a file and save it you can recover the correct version of the file, even if it goes back several versions. Time Machine makes this process completely intuitive (as well as pretty!), and extends this capability beyond just the file system. Need to recover a old version of someone's Address Book entry, or an entry that you've deleted entirely? Not a problem!
Core Animation is a new rendering engine optimized for the new Intel processors, and designed to take full advantage of multiple processors and multiple cores. It's under-the-hood stuff, and of direct use to developers (WWDC, go figure), but check out the freakin' demo! It's (I'm guessing) a screen saver that pulls images from an iPhoto library and does amazing shit with it. I can just imagine this with my 'Con photos! ;-)
I'll be interested to see what other changes/enhancements Leopard has. I'm hoping for some Finder improvements.