is a great Photoshop file, with several layers, each of which you can view on your monitor as an aid in determining how well your monitor is calibrated and profiled.
Free, but requires an email sign-up. I joined them several years ago, and get an email once every blue moon.
(Oh, and if you're of a mind, join and make a contribution while you're there...)
I've seen, and probably have, some variations on this, over the years, but here everything is in one nice place. Links to similar useful files would be appreciated in replies, if you have them.--
Tracy Valleau, moderator