It doesn't even have to be someone credible.

Simply puting some images of human faces on your site near the call-to-action button helps. The Facebook widgets that show who has "liked" your site work great for this. Our eyes are drawn to pictures of faces and a group of faces next to a button says "these people tried it, what is wrong with you?"

Some more: * Pictures of faces * A testimonial (one face and one sentance) * A number indicating usage (4,243 downloads today!) * Expert reviews (TechCrunch loved it!)