Elasticsearch is a real-time distributed search and analytics engine. It's used in projects of any size from startups to huge enterprise search clusters. Some examples include:
Wikipedia. It uses Elasticsearch to provide full-text search with highlighted search snippets, and search-as-you-type and did-you-mean suggestions.
The Guardian. This media outlet uses Elasticsearch to combine visitor logs with social-network data to provide real-time feedback to its editors about the public's response to new articles.
Stack Overflow. This online project combines full-text search with geolocation queries and uses "more-like-this" to find related questions and answers.
GitHub. Uses Elasticsearch to query 130 billion lines of code.