I don't have a whole lot of screen real estate, so I like to use tabbed browsing while keeping a single Safari window open at full size. The problem is that this makes it difficult to get at documents behind the browser. Expose isn't bad and Command-Tab helps, though Hide and Minimize are less useful (personal preference).
In Safari 1.3, the Debug menu (Enable the Debug Menu in Safari) contains a new "Use Transparent Window" option. Now, to see behind the browser, all that is required is to switch to a tab containing a blank, transparent window (prepared in advance). Since the opacity is below some threshold, it is possible to click through the window, and by using Command-clicks, to access background windows and UI elements without changing the frontmost application. The tab switching itself can also be performed by Command-clicking while keeping Safari in the background, as opposed to un-hiding or maximizing, which would always force Safari to the front and end up covering everything else. I guess I could always buy Windowshade X, but for my workflow, I only need it for Safari. Besides, this is built-in and free.
[robg adds: I'm not sure why this feature was added, and it's a bit odd in its behavior. If you have a page loaded, the transparent window setting will just basically make the window's shadow disappear. But as noted, an empty page will completely vanish. And if, like me, you use the status bar, you're left with this very odd window: header area, large invisble section, and a status bar floating below that. Quite strange...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050418015445258