This is a nice way to ease into the week (courtesy of an afternoon article in NetTuts+). Rather than using boring solid-color boxes as placeholders in your website mockups here are two (cute!) alternatives: placekitten (my personal preference) and PlaceDog. For either one you just put your image dimensions (width & height in pixels) after their URL and you’ll get a placeholder image (so something like: http://placekitten.com/200/160)
PlaceKitten Sample (200x160)
PlaceDog Sample (200x160)
Or for a slightly more serious version, you can pull themed images (based on tags) from Flickr with flickholdr.com
FlickHoldr Sample (200x160, ocean)