ShareDB is described as 'Realtime database backend based on Operational Transformation (OT)'. There are five alternatives to ShareDB, not only websites but also apps for a variety of platforms, including Self-Hosted solutions, Node.JS, Windows and Android. The best alternative is GUN, which is both free and Open Source. Other great sites and apps similar to ShareDB are RxDB (Free, Open Source), FeathersJS (Free, Open Source), PouchDB (Free, Open Source) and Yjs (Free, Open Source).
Yjs is a CRDT implementation that exposes its internal data structure as shared types. Shared types are common data types like Map or Array with superpowers: changes are automatically distributed to other peers and merged without merge conflicts.