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 above, how in the iOS Mail app, the "Drafts" folder icon is a partly folded paper airplane, and the "Sent" folder icon is a folded one:

Or in OS X, where when you enter dictation mode, your computer's fans automatically turn off.

Check out the blog: http://littlebigdetails.com/