does this exist?

Is there such a thing as one of these: http://todbot.com/blog/2008/02/18/wiichuck-wii-nunchuck-adapter-available/

only for a USB cable, like the kind in webcams? what about for a headphone jack that are attached some webcams?(the kind with speakers)

In short no. You can get a USB to I2C adapter http://www.robot-electronics.co.uk/acatalog/USB_I2C.html and wire them together but the nunchuck will need a voltage regulator in it as well as a signal level converter for the I2C lines.