First, remember that STEEM is the blockchain on which several apps are built. First, there's Steemit.com, which is the main front-end to the blockchain, however, there are many other apps and projects that have been built on top of the STEEM blockchain and allow everyone to earn rewards to their Steemit account.

These include DLive.io, DSound.audio, Utopian.io, Musing.io, Busy.org, Steepshot, ESteem, DTube, and many others.

It is important to remember that all of these allow rewards to be generated in STEEM. If you're looking for other platforms like Steemit, I'm not sure there are many that are active so far. There's one I've heard of called Sola.ai, but I'm not sure how successful it has been.