You already knew Safari had been released for the Mac, but this is just in from the Apple Developer Connection newsletter:

In addition to providing the best web browser for Mac users, one of
the goals of Safari is to provide a fast and efficient HTML
rendering engine for Mac application developers. Apple is actively
preparing a Safari SDK that will be available later this year.

Now that's legitimately big news. If Apple can do for OS X what the Microsoft Internet Control does for Windows programmers, namely to create a full-fledged, mature, embeddable browser component, the possibilities for new applications jumps like mad. It's hard to find a Windows client application these days that doesn't take advantage of the Internet control, just take a look at the applications' sys reqs. They almost always require IE 4+ these days, which usually means they've used some of the embeddable control's framework.