OS X 10.4 Tiger
Awesome! Just upgraded my os to Tiger from Jaguar recently. I have to say, it was well worth it. The upgrade process took about an hour, maybe less. Because I chose to do a clean installation, it took longer of course, compared to an upgrade. Your system will be up and running in about 15 minutes for an upgrade. The significant factor I noticed first was how it ran faster. Before with Jaguar, I hit the power button and it would take a while to boot up. Now, it takes a couple of seconds. NICE! Now I don't know if it's because I did a clean install, or because I upgraded to Tiger, but my mac is just more responsive and fluid.
Another cool feature that comes with Tiger, is the dashboard. By default, you get widgets that give you a weekly weather forecast, a worldwide clock, calendar, dictionary, etc. You can download a whole lot more at the Apple website. I downloaded a widget that shows you any open/encrypted wireless networks. I've used konfabulator before and I kind of miss how the widgets just sits on your desktop at all times. I guess I'll adapt to the new way of how I hit F12 to open dashboard and hit it again to make it go away.
A second feature I like is Spotlight. It searches your entire hard drive for any type of file you're looking for within seconds. I've been using it, and I was surprised how fast it was.
The new Safari is pretty too. They put in new features like built-in RSS feeds. It's convenient how they added exporting/importing bookmark feature. They definitely caught up with Firefox. I don't use Firefox any more since there's all the features I need with Safari. I still love Firefox and still use it on my windows computer.
Quicktime is another upgraded app with a new video codec called H.264. I checked out video clips on the apple website and they look awesome. I have a Powerbook G4 w/1.25Ghz and I was able to run the videos of 480p and 720p but my mac couldn't handle the 1080p. You'll need the Powermacs or the Imac G5 maybe to run those. LOL. As I mentioned upgrade to Tiger was worth it, any apple computer I think is one heck of a machine. Occasionally you get errors but that goes for any computer. Macs are very stable and everything just works. If you do plan on buying a new computer, go with a mac. You won't regret it.