Mark James decided the Internet didn't have nearly enough cats on it, so he invented Placekitten. Placekitten was created so that web designers and coders could have a cute placeholder for their work. Instead of a traditional image holder, the site lets you quickly and easily plug in a picture of a cute and cuddly kitten.
Placekitten has a variety of cute kitties to choose from and is virtually guaranteed to make missing code or segments of a page look cute as opposed to ugly or broken. What's great is that you don't need to be an expert programmer or web designer to insert kitties into your work. Placekitten provides the URL and all you need to do is specify the height of the image.