Google Custom Search - Google Custom Search enables you to create a search engine for your website, your blog, or a collection of websites.
Meta Search - Search your Desktop, Google Drive, Dropbox, Gmail, Evernote.
Klevu - Klevu offers instant site search solution for eCommerce stores.
Findo - Your smart search 🔍 assistant across personal cloud ☁️
ElasticSearch - Elasticsearch is an open source, distributed, RESTful search engine.
FYI - Find your documents, like magic 🔮