What are some excellent examples of single pagers/forms to validate demand?

I keep hearing to validate demand before proceeding to develop product & design.

This is great in concept but I've not personally seen any examples of it working.

I know there are a few here that have done it successfully. Using facebook ads/communities to drive traffic to a holding page that promises a great product.

But are there any examples of this working? To me, unless I see screenshots of a functioning product I'm not likely to part with my email address let alone actual cash.

Are there any demonstrations of an effectively designed landing page with the style of copy/positioning that has worked in the past?

Thanks!