We covered Mashape last fall in our overview of 11 API management tools. But Mashape is a different sort of management company, with an open listing of APIs and the ability to buy (and therefore, sell) APIs.
"When you're a marketplace, you always fight the chicken/egg problem," Mashape's Augusto Marietti said. "The last thing you want to do is open up a marketplace when you don't have enough supply yet." Mashape now lists over 400 APIs.
Perhaps the most interesting part of Mashape is that it enables anyone to create an API. Granted, you still need to do much of it on your own site, but it can be proxied through Mashape, so it's available to developers using the company's marketplace.
Some developers might question the latency of using an intermediary, but the benefits could outweigh the milliseconds. Mashape lets developers have a single credit card on file for billing, similar to how Google Console works for Google APIs.