If you use Safari, either on Tiger or Leopard, you'll notice some differences. If you're reading this in Safari right now, you'll definitely see some differences. I had to go into Safari's innards to fix an interface annoyance, and while I was there, I decided to have some fun.
To change your windows so they don't have that "oh look it's high-tech" brushed-steel texture thing going on, do this:
sudo open /Applications/Safari.app/Contents/Resources/English.lproj/Browser.nib/
And then go to
Tools > Show Inspectorand do this:
Voila - the "cool" brushed silver is gone and the "awesome" drop shadow is gone too.
You can take it further than that. Do this:
ls -laF /Applications/Safari.app/Contents/Resources/*.tif*
...to get a list of all the TIFF-format images used in Safari, the app. If you change all of these images and keep them at the correct size and color depth, you can probably create an entirely new visual theme for Safari if you want. I say "probably" because these edits don't always work. One thing I absolutely hate about OS X is the Help menus, so I tried to remove Safari's Help menu, and unfortunately this destroyed something important, somehow, because after that Safari refused to boot. (Luckily Interface Builder backs up your interface files when you edit them.)
However, this does bring us to the other thing you need if you're going to edit Safari: a backup Web browser.
I like Shiira the best. Geoff Grosenbach used it in a Peepcode screencast, and it's got a happy whale. The UI is also pretty cool, and smart, and comfortable, and there's even a Dashboard Widget version, but honestly, it's all about the whale. Or maybe the fish. Looking at the tail, I think it's actually a fish. Either way, it's definitely happy.
Camino is also good. I use Firefox for dev, of course, but its cursor behavior truly annoys me, so I avoid it for anything but dev. Before I figured out how to fix the interface annoyance that started this all off, I thought I was going to have to give up on Safari entirely, so I checked out some other browsers as well - Opera, OmniWeb, and Sunrise. Sunrise has the most original thinking and the least stability. I didn't really dig Opera. OmniWeb's marketing annoyed me so much that I never really spent much time on it.