Probably my favourite add-on for the PS2 has to be this, I am now looking for a replacement since , after 3 years usage, mine has stated to fail.
The Mad Catz Gametrak Controller Gametrak is a revolutionary videogames control system , which captures your movements and puts YOU in the game!
Gametrak tracks your movements in full 3D with no time delay.
Compatible with PlayStation 2, PC and XBox, Gametrak lets you punch your opponents with your hands and gives you sports games that let you pick up and play using real golf clubs or tennis racquets!
When you combine this with a game called Dark Wind it is just a fantastic system, works with golf too, but I can't recommend the golf myself since I have never played it.
£2.42 with Amazon Prime, or add Dark Wind for free postage for all (£4.99)....http://www.amazon.co.uk/GameTrak-Gametrak-Dark-Wind-PS2/dp/B000I6KI12