Likely Ebay & Youtube were the firsts of their kind, after that, threadless & lulu became the next ones that I personally had heard about. (the most recent, is veer)
The uprising of ideas of this nature is fantastic.
It allows every day joes to create passive income opportunities, and people with innovative ideas to become millionaires.
The concept is this:
Create an niche idea that would require a huge amount of content.
Say, a collection of photos of angry people yelling; or people getting splashed by cars as they drive by.
The more creative the better.
Next, create a portal for people to submit their content, and promise financial reimbursement when clients pay to use the images (often different costs for amounts of usage)
Next, market this portal to:
A) People who would have the content to submit
B) People who would want to purchase the content
Passive and beautiful, the archive takes care of itself. Hire people to screen/rate/price content, and update the site as necessary.
Which begs the question: What does the world need a lot of, that a niche of people could provide...?
Sensei Ono, Shinka Martial Arts