Hi Rand,
I'd like to add amateur video testimonials to the list. Dove started the trend offline by getting actual customers to review the product in their ads and the same concept seems to work quite well online. One of our clients mailed out disposable cameras to their clients and had them record testimonials which were then used in a variety of campaigns including landing pages, email marketing and on their site. Led to an increased engagement as the clients themselves shared the website link where their testimonial was hosted with their friends/acquaintances. Not sure if this qualifies in the "very few people know of" category however i thought I'd still share it.
Cheers,