ADSB and Opensky are the big ones. IIRC one of them censors aircraft on the owners' request but because they're lower profile lots of planes that are blocked on Flightaware are still visible. One of them also has an API that returns "interesting" aircraft (military, government, owned by celebrities etc) which is fun to look at

Edit: I think it's Opensky that filters their data. Not many planes are filtered but their coverage also isn't as comprehensive as ADSBExchange. For one specific usecase I did find Opensky's API easier to work with