Buzzfeed discovered this week a hidden feature on Facebook which allows you to see who has left your friend requests hanging.
To access it, you simply click on the friend icon at the top of your Facebook page (the spot where you accept all those new requests), then select 'Find Friends'.
A page will load with all the people currently waiting to become your friend. At the top, there's a tiny "View Sent Requests" button - click that, and you'll see what requests you've sent that haven't been approved.
If someone deletes your friend request rather than just ignoring it, they won't appear in the list.