Where can I download the versions of Safari that are newer than what Apple ships? I'm a web developer that makes webapps that work in all modern browsers. Amazingly enough, I use 100% valid HTML5, CSS3, and JavaScript. Occasionally, I have to release interesting fixes, to say the least, because my webapps don't work in the latest versions for some reason.

Thus, I have the latest versions of all browsers as well as the betas. Firefox Aurora, Chrome Canary, IE10, and Opera Next 12.

I have but one missing: Safari. Of course I have the normal version of Safari, but can I get ahead of the curve before the launch of the next Safari update by Apple?

I'm looking for either the Mac or the Windows versions - both if possible.
Safari is Apple’s build of WebKit. Nightly WebKit builds are available at nightly.webkit.org.