Checkered periwinkle, Littorina scutulata.
There are about a dozen of these in my aquarium, some barely a pinhead across, some full-sized, like this one. Most probably hitchhiked in on a handful of seaweed or eelgrass.
These snails live in the upper intertidal zone, more out of the water than in it. They can even drown if they stay underwater too long. But they are more versatile than most of the intertidal critters; they tolerate dry rocks, salt water, and even fresh water, for brief periods.
In my tank, they hang around the lip, just above the water level. Sometimes, one works his way out from under the lid, and rolls off the tank. I have found them several feet away, on the carpet, dry and sealed up tightly. One tiny, tiny snail was under a cushion on a chair; I have no idea how long it had been there. It was so dry, the shell was grey and rough-looking. I dropped it in the tank, and it woke up, stretched, and started the journey up to the lip again.
Every so often, they head down, like this one, looking for a meal of algae, which they scrape off the glass. In the photo, the big white circle is the base of the foot; the smaller circle, in front, is the mouth. Look closely, and you can see a hint of the radula, like a tongue with teeth on it.
Update: Click on through and read Christopher Taylor's comment. Astounding!