Algolia - Algolia's Search API makes it easy to deliver a great search experience in your apps & websites. Algolia Search provides hosted full-text, numerical, faceted and geolocalized search.
Meta Search - Search your Desktop, Google Drive, Dropbox, Gmail, Evernote.
Google Custom Search - Google Custom Search enables you to create a search engine for your website, your blog, or a collection of websites.
Vaultedge - Access all your documents in one place.
ElasticSearch - Elasticsearch is an open source, distributed, RESTful search engine.
Google Cloud Search - Search across all your company's content in G Suite.