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.

If you are registered as a Safari Developer (which is free for anyone), you sometimes get access to beta builds of Safari. For example, developers currently get access to a beta of Safari 5.2, the version shipping with Mountain Lion. Think of this as the beta channel of Chrome - where its much more stable and almost OK for daily use. I am not sure if the beta is available for Windows right now, but they usually do offer a beta build closer to release time (this is also usually too when its no longer a private beta, but a public use beta). The difference in these beta builds are usually more user-oriented new features, etc.

Safari (and Chrome, and a few other browsers) are built on WebKit, and you can download nightly development builds on their site. Think of this as the Chrome Dev channel, or moreso Canary with nightly new features (and bugs). The focus of this version is nightly bug fixes or new features.