I don't know their names (if they even have official names), so I made some up. At least five of the seven "medallions" have a function.
There is an "official" walkthrough that also describes these items, and it comes to the same conclusions about the powers of each item. I've added (in parentheses) the names that the walkthrough's author gives each item.
From the left to right:
Bird (Invisible Platform) medallion - Looks like flying bird. Gives you the ability to detect where invisible platforms (you'll see blinking blocks) are located.
Plant (Claw) medallion - Looks like those (hidden) plants that eat you. Gives you the ability to see (you'll see a slight sparkle) those (pesky) plants, so that you can avoid being eaten.
Fish (same) medallion - Looks like a crescent-shaped fish with spikes. Makes one of the fish in the giant lake follow you.
(??) medallion - Looks like a cave or a shell. I have no clue what this does.
(???) medallion - Looks like a sailboat or something. I have no clue what this does, either.
Snake (White) medallion Looks like a snake in the upside down U shape. Lets you see where the hidden white orbs (white flashing dots) are from the map screen.
Star (same) medallion - Looks like a star. Gives you the power of the "W" (key)!