If you like using List View in the Finder, you're probably familiar with the fact that you can use the disclosure triangle next to a folder to see its contents in the current window.
Here's a neat trick with that: If you Option-click that triangle instead, it'll open the folder as before, but Finder will also expand every single folder within it. So you can see the entire contents of a folder—and all of its subfolders!—with one single click.
I find this especially handy if I need to organize the contents of a folder, as it's faster to move stuff around this way rather than opening separate Finder windows. To me, at least.
To collapse everything back down, just Option-click that disclosure triangle again. If you don't do it that way, all of the subfolders will stay uncollapsed until you do. Which you may enjoy if you're all crazy like that.