Jan from work recently gave me a huge bag full of grapefruit from her mother's tree. They were very gratefully received, and my first thought as to what to make with them was grapefruit marmalade.
After flicking through a few recipes, I chose to make Nigella Lawson's pink grapefruit marmalade from p351 of How To Be A Domestic Goddess, using ordinary grapefruit rather than pink grapefruit. You can find the recipe online here. This recipe won me over because it does not require the fruit to be soaked for 24 hours beforehand, and the whole of the fruit is used, pith and all.
The finished product is a rather bitey marmalade, despite the 1kg of sugar used in making it. I assume that using the pith as well as the flesh and skin of the grapefruit added to the tartness. However, if you like your marmalade to really zing, this is a good recipe to use, and it is a no fuss way to make marmalade.