Here's one that a designer friend told us about, so bear with us, because it is a little bit niche. But if you’re in the creative industries you might be interested in lorempixel.com simply because it's so easy to use. And even better, it's free.
An essential part of designing any successful layout, be it print or web, is getting the images in the right place and here you can get placeholder images for any kind of creative project you're working on. Input your size and desired subject matter and off it scurries to find you a suitable filler image.
Much like the place holder text from which this site takes its name, these aren't meant for the final job, they're used to show how a final design might look, so they're low-res and designed to be removed when you get the final, proper images and, more importantly, when you get the client’s sign-off.
It's as useful a tool for designers and developers as we've seen in quite a while