What is the Windows hotkey to minimise a single, currently active window?

I've been using the shortcut Alt+Space followed by N for years. Works on any Windows version, all the way back to Windows 3.0! It should work with all keyboard layouts and probably even all Windows localizations.

Alt+Space opens the general windows menu. You can open it manually by left-clicking with the mouse on the top left window corner.

The menu contains the options to maximize, minimize, close the window, etc. It also underlines the key you need to press to select the appropriate option ("miNimize").

The N is not a general "hotkey" but an access key – a keyboard shortcut to an entry in a GUI menu that works only if/after the menu opens. The menu items texts differ across localizations (languages), which is why a different letter might be underlined and serve as the access key.

A small workaround is Alt + Esc – it doesn't minimize the active window but places it behind all other windows. The effect is similar however.

I know it's not precisely what the question asker wanted, but it's a built-in Windows one-step solution that works in a similar way.

In Windows 7 and later you can minimize the currently active window with: Win+Down.

(This may be an Aero feature that works with Vista as well.)