You can download nightly builds of WebKit so you can try it out. But its mainly for like… idk how to say this… um… well, testing. You can test it and see which version of WebKit works out for you. Oh wait, I forgot to mention this: You need the latest version of Safari in order to use these nightlies!!! WebKit is not a stand alone browser. Its a web browser layout engine, and Mac OS X uses WebKit extensively in many apps, like Mail, Dashboard and whatnot.
Heres the link, and read more about it:
One last thing i forgot to mention, I don’t think these nightlies will work on anything earlier than Mac OS X 10.4.11 Are they universal binaries? I don’t know really. I only have a PC and a old iBook with 10.3.9 installed, so I can’t test it on my iBook 😦 but i can test it on my pc, I guess….