I’ve talked to a few people who’ve updated their Mac to Leopard who’ve not really seen much “new” to appreciate in Leopard. This won’t change your world, but here’s one. If you’ve used Chax, a third-party upgrade to iChat for older versions of OS X, you’ve already had tabbed chats, but now Leopard users get them built into the system. You can see an example above of what they look like.
When you have iChat on, open your Preferences panel and click the Messages tab. (Click the image to the left to see a full size image of what to click.) At the bottom of that panel, there is an option that you need to check called “Collect chats into a single window” and you’re set. The next time you’re chatting to multiple people, the windows will be tabbed like you see them above. Handy.