Canon lenses with USM auto focus can be called "continuous manual focus" (at least some). By this I mean that you can override the AF without switching it off. I've found this very handy, and I often miss this option when it's not there.
I know Sigma have HSM AF on several lenses, but I've encountered at least one of these which still requires me to "turn off" AF with a switch before I can manually focus. Does anyone know where I can find a comprehensive list over lenses from Sigma (and others) which have "continuous manual focus"?