A few open source options exist (Spotify's Annoy, Facebook's FAISS, NMSLIB) but these are rather low-level, more hobby projects than enterprise level engines (index management, transactions, sharding…).
After building a few document similarity engines for our clients we took up the gauntlet and created ScaleText, https://scaletext.ai. It's still early days but the demand for a sane, scalable, cross-vertical and well-supported NLP technology is encouraging.