You can favorite items in the Shop or in My Stuff by clicking on the heart below the item viewer. Games can also be favorited by using the favorite heart on the game info page in the games tab. If you have already favorited something, the heart will be reddish-pink. A list of your favorites is available in your profile and can be viewed by others. You can unfavorite an item by clicking on the red heart to switch it back to the empty heart.