I installed Tiger, the new version of Mac OS X, last night. I had just enough time to glance over it and drool a bit before I went to Friday Night Waltz (an extra one for National Dance Week) but I've gotten to play with it a bit more today. I'm definitely liking it so far. I'm completely sold on Spotlight as a Launchbar replacement. And having Spotlight in Mail.app along with the new Smart Mailboxes seems like you could get it to behave a little more Gmaily, which would be cool. (Still, it's not quite the same as having labels, though. Too bad.)
For sheer, widgety fun though, the Dashboard is the most addictive new toy. It's full of customizable (not to mention very pretty) little mini-applications that popup and disappear at just the touch of a key. Some of the defaults are very useful (like weather forecast, dictionary lookup, stock prices, phone book, etc.) but there are also lots of extras you can download. Some of the ones I like are a wikipedia.org lookup, and a mini-wiki, which is like sticky notes on steroids. There's also, of course, DashBlog for posting to your Blogger blog. No fancy stuff at all, but very quick and easy for short posts, if you do a lot of that kind of thing. I was a little bit bummed about the default iTunes widget, since you can't control it from the keyboard, so I'm not quite ready to give up X-Tunes for that yet. But all in all, I could probably spend ages playing with the Dashboard and all the possible widgets. It's just fun that way.
Some things I haven't gotten around to yet but that look like fun: Creating new Dashboard widgets. Writing Automator scripts (maybe there's a good way to get all of my old archived email from Mail.app to Gmail). Safari RSS (not that I need another feed reader, but it's cool to see a browser-intergrated approach to it). And probably lots of other things that I'll continue to discover as I play with it more.