What do you use for placeholder assets?

I see myself in front of a virtual word... which is still very empty. Frankly, a plane and a few boxes. I want to put some placeholders in there.

I know that I am not an artist, and I am lacking the necessary skills for some of those things I need. Also, I want to try out stuff quickly, without making assets which I will throw away anyway because I decided against a specific feature. Then, I want to show the whole thing to people who can create the necessary assets for me.

I am pretty sure that most (if not all) game developers hacking around have that very same problem. So what do you use in place of your final resources, and where do you get your placeholders from? Simple meshes, textures, sounds, etc, maybe not even high-quality, just so that you can put something there....