[user-id]

http://lorempixel.com/ is what I use.

[Michiel]

Let's not forget http://placesheen.com if you need placeholders of Charlie Sheen.

[poissonpie]

As a little self-promotion, there's also http://www.placeguitar.com

edit....just to link the source code in case anyone is interested https://github.com/poissonpie/Place-Guitar