Webkit is the open source engine used by Safari, Dashboard, Mail and many other Mac OS X applications. You can download nightly development builds of the project. The current version is the fastest browser avilable on the Mac. Even beating Firefox 3. Some tests indicate that it is two and a half times faster than the current version of Safari.
Keep in mind that this is a development build and may not be entirely stable. If that is a concern then wait for the official software update from Apple. But if you cannot wait then you can download it now. It looks and feels just like Safari, except it is called Webkit. It is available for Mac and Windows.
While it does not have the add-ons that Firefox offers it has the speed. So much so that if I do not need a Firefox extension or compatibility and just want to browse the web I turn to Safari/Webkit.