This work-in-progress document will list all of the ADS-B services from around the internet (as well from the OARC tracker!) that you can feed data too. They all have advantages and disadvantages according to services provided, what you get for feeding, and your moral choices (as you'll see).
Links are provided to where you'll find install information for feeding each website, although you'll have to pick an appropriate path depending on your setup. For most people this will be readsb + individual install scripts for each website, rather than using a full image from a particular website like FR24 - it's the best way to manage things.
This side of the community can get a little weird. People fall out with each other, projects split, projects merge. Here's the current state of the open-source replacements to ADSB Exchange after that site got sold and what the various people involved created. All use the ADSB Exchange software stack and support MLAT.
The OpenSky Network is officially an academic research project. However, it is supported by some defence companies as well. They don't support MLAT and have a custom feeder client.Instructions
ADSBHub is a community-driven aggregator that has its own custom feeder client but no MLAT support. Instructions
TheAirTraffic is run by the chap behind the ElonJet service who were famously tracking Elon Musk's jet. It uses the ADS-B Exchange codebase and supports MLAT. Instructions
PlaneSpotters is a civil aviation database with a focus on photographs and registration info. They are based off the ADSB Exchange software stack and support MLAT.Instructions
This site seems to make it incredibly difficult to sign up to feed them, and they're a pretty small site. Your call. Instructions