Petzl William Triact-Lock
The William Triact is the twist-lock version of the William series. These are HMS carabiners, designed to take a Munter hitch easily (with a name like "William", it is clearly designed for us folks named William ;) ). The locking sleeve is used by lifting then twisting the sleeve. I find this action a bit awkward to do one-handed, at least for my size hand and the William-sized carabiner. But the sleeve locks automatically and positively when released, with a satisfying "click" (you still want to give a squeeze check, of course, to any locker).
The main uses I have for the Triact are in setting anchors in instructional situations (the locking mechanism deters casual playing around by students, especially youth) and in a pair with my Silent Partner. In most other situations where I want a locker, I prefer the William ball-lock or a screw lock.