Craft with ElasticSearch as a backend
Although not related specifically to Craft, we built https://www.jelliscraig.com.au website using CraftCMS with all property listing data is stored in ElasticSearch. We use a simple plugin for the ElasticSearch PHP client but the integration is front-end only, for the most part. 
We have been testing pushing Craft generated content into ElasticSearch via the element API and in theory its a workable solution however for our specific situation we see no huge benefit in storing all general content externally.