The one's with known effects are as follows:  
Cancer - faster firing speed  
Counterfeit Penny - gives a chance of one extra cent being collected upon picking up a coin (dimes might be 11, nickels 6, pennies 2)  
Goat Hoof - Randomly causes a tear to increase in damage  
Burnt penny - chance of a bomb spawning upon picking up a penny  
Missing page - When using a spacebar item a random book is used, and chance of activating necronomicon when damage is taken  
Bloody penny - Chance of dropping half heart upon picking up a coin  
Lucky Toe - chance of extra items when opening chests or from blue rocks  
Isaac's Fork - chance to regain health upon kill  
Liberty Cap - Random odd mushroom effect each room  
Push pin - Chance that a tear becomes "spectral (Ouija Board effect)  
Tick - Boss health decreased, cannot be exchanged  
Umbilical Cord - If brought to half a heart Little Steve will be spawned  
Safety Cap - Identifies pills  
Rusted Key - Chance to drop a key when hit  
Pinky Eye - Chance a tear will be poisonous  
Red Patch - Chance to activate the "Razor Blade" when hit, and sometimes shears  
Mom's Pearl - Chance to freeze enemy