There’s a Tumblr blog called Little Big Details that features just that: small user-interface and user-experience design details in Web products that make a big difference.
Like here, how in the iOS Mail app, the “Drafts” folder icon is a partly folded paper aeroplane, and the “Sent” folder icon is a folded one:
Or this one, from OS X, where when you enter dictation mode, your computer’s fans automatically turn off:
Or this one from Facebook, where the icon for marriage actually matches the genders of the married couple:
Or how in Pinterest’s iOS app, scrolling down hides the top bar and scrolling up makes it re-appear: