Google Chrome for Mac (well, Chromium)
Google Chrome currently isn't available for Mac OS X.
However, you can download Google's open source project, Chromium, and build that. And, I have to say - it's pretty freaking slick.
I'm currently writing this post in Chromium under Mac OS X. It's working fairly well and looks fairly promising. You can rip off tabs, and it does the same neat window effect that it does under Vista. For a test build, I've only had it crash once on me.
So, looking cool.