The challenge when exploring this topic: the incentive to stay under the radar. Those succeeding don't have much to gain from sharing details here. Worst case: it could invite competitors into their space.
Communities that thrive on growth (e.g., open-source) tend to share freely, but API businesses, especially ones which are easy to execute, often guard their edge.
A recent finding I had, while not necessarily an API: services which help you 24/7 stream a lenghty video file. YouTube live streams seem to work well for those lofi-types of channels, and there are services which are built to enable autopilot live streams.