Open every subfolder in List view

In List view, ⌥→ expands a folder and everything inside it. ⌥← collapses it again.

How to do it

1. Switch to List view (⌘2).
2. Select a folder and press ⌥→: the folder and every folder inside it expand.
3. Press ⌥← to collapse them all again.
4. Without Option, → and ← expand or collapse just the selected folder.

Shortcuts

⌥→⌥←

More detail in the guide: Keyboard shortcuts