The only usb key I have encountered with U3 on it was a sandisk.Every other kind I have is just a plain simple USB key. So I would saymost are simply USB keys. If they have U3 or similar they tend to advertise it as if it was an amazing and useful feature.
Useful links for getting rid of U3... http://en.wikipedia.org/wiki/U3 http://www.u3.com/uninstall/final.aspx