There are different brands of photocromatic lenses, however none match up to what Transitions offers.
Transitions lenses are lenses that change from light to dark. Most do not change behind the windshield of a car, though.
The newest addition of transitions is Transitions 7. These lenses are virtually clear indoors; when outside they change to a comfortable grey or brown color. They also change the fastest of any Transition lens.
The other two main types of Transtions are XTRActive and Vantage.
The XTRActive are the darkest indoors and change to the darkest outdoors. They also change a bit behind the windshield of a car.
Then there are the Transitions Vantage. These lenses have a slight color indoors, however when in bright sunlight there are molecules that hitch together and form a polarized filter. Vantage lenses also do not change behind the windshield of a car.
While Transition lenses are convienient, they still are not a replacement for a good pair of prescription Polarized sunglasses.