Ordnance Survey offers a free desktop wallpaper image each month.
The wallpapers feature superb, beautiful and scenic wallpapers repated to fantastic sights which can be seen on or around in Great Britain.
What I find particularly useful is that the current month calendar is superimposed discretely onto the image. Useful for those who need to refer to calendar dates a lot.
I know its not much of a freebie, but its free nonetheless!