With a little help from Bob from Super User, I finally managed to implement a userscript today to add a feature to Amazon that I have been waiting for for far too long.
The script adds the ability to label your credit cards that are stored with Amazon.
On the checkout page, during payment selection, you'll see a new option to label the card. The label is then displayed next to the credit card the next time you're at the checkout.
If you're wondering what the point of this is, then you probably only have one credit card. If you have multiple cards, from the same issuer and all in the same name, then you only have the last 4 digits to identify your cards. This is painful and annoying.
…especially if you have multiple cards ending in the same digits :P