Ever since I got a mac I missed the functionality of "show desktop" that windows has, with its keyboard shortcut: windows key+D.
F11 simply doesn't cut it, creating messy borders at the edge of the screen.

This is my goal: hiding all windows using a keyboard shortcut.

Step 1: Downloads

you'll need to download 2 freeware applications, and install them:

1) Show Desktop
This will hide all the windows
http://www.everydaysoftware.net/showdesktop/

2) Xkeys - 1.1.1
This will allow us to use function keys.
http://www.versiontracker.com/dyn/moreinfo/macosx/18713

Step 2: Using Show Desktop

Open Show Desktop.

When you press on the Show desktop icon in the Dock it will hide all applications (depending on the preferences).

go to the preferences in Show Desktop, and see the different options, this is something you should do with every new program you get so as to use it the best way you can.

I set my prefrences this way (see picture):

Step 3: Using Xkeys

open Xkeys

see image 1, and copy it.
Press F8 (or the function key you chose), this should hide all the windows!

but wait, we're not done yet,
remember about always checking the preferences?
Go to the preferences (see image 2) and uncheck the last box (the one that says can put Xkeys on standby using...) so it does not collide with other programs.
For example cmnd+F12 is used by firefox as "undo close tab" but will be overridden by Xkeys, creating much angst and dismay!

That's it, you're done!