Thanks. With ND grads, I find myself using the ND4 most (reduces light by 2 stops) because this is the most versatile. I also have an ND8 which stops light by 3 stops, which is what I used for the London skyline. The ND4 is a medium so the transition between dark and light is in the midline, and then the ND8 is a soft, which is a progressive graduation all along the filter. The ND4 is the more essential from my experience, but as TelexStar has quite well illustrated, it all comes down to a matter of photography style.
Although Cokin themselves don't make the best ND grads (there is nothing wrong with the Cokin ones, but I would recommend Lee for their quality; however they are many times more expensive), their filter holder has become very popular and other brands make filters to fit into this. The advantage of such a system is two-fold - the thread size as we've touched upon, and with ND grads this is useful as you can alter where the transition line on the filter to line up with the horizon, which you can do with a screw-on.
You will find a listing of the Cokin ND grad filters here:
And by-the-bye, it seems we've moved away from the theme of blurring water. Should we start a thread for ND grads/NDs?