ship spotting
Popular services like marinetraffic.com or vesselfinder.com do rely on numerous AIS station around the world.
Volunteers are collecting AIS-data within reach of their receivers and pass them to the servers. There are still areas, not yet covered!
Setting up a receiving station is simple and does not cost a fortune: Internet access, raspberryPi, a cheap DVB-T-stick and an old TV arial do the job.
The owner of an AIS station benefits from extra services offered by service providers, others have to pay for.