- http://placehold.it/ - Simple image placeholder
- http://placekitten.com/ - you can guess from the url :)
- http://lorempixel.com/ - another images placeholder service
- http://placeboobs.com/ or http://worksafe.placeboobs.com - NSFW Image placeholders
- http://dummyimage.com/ text & background-color
- http://placeskull.com/170/170
PS: Good part of image placeholder services is here: