Steem Monsters also used custom_json. Reportedly, Steem-engine is/was a side chain, but I never learned much about how that was implemented.

Steem Engine uses the Steem Smart Contracts software created by @harpagon. The software scans all blocks added to the Steem blockchain for transfers, custom_json, or even comment operations that contain a certain sidechain ID. It then executes the smart contract function specified in the operation data and packages it into a block which is added to the sidechain. It also maintains the current state of the sidechain and all smart contracts running on it which can be queried using a familiar RPC API.

Also, there was pocket - and of course SMTs were supposedly ready to launch right before the Tron buyout and the subsequent controversy.

So much untapped potential, if someone could just harness the right mix of funding and skills. Hopefully, @marcosdk's initiative gets things moving in the right direction again.