Trust is the new currency of the global economy. It makes or breaks sites while cementing the relationships upon which the strongest brands are built. By virtue of having so many facets, trust acquisition is almost an art, but we really do know that it's more of a science.
As such, it relies on a lot of visual and support elements such as a clean and fast interface, clear language and inobtrusive opportunities for human interaction. But it's also about assurance, and the amount of perceived safety offered by a good site translates directly into the warm and fuzzy feelings that visitors want to take away and share.
One site that offers such warm-fuzzies is Kiva.