For example: https://www.google.com/#q=related:http:%2F%2Fagiliq.com%2Fbl...
It returns pages related to any given page, and works at the specific page level (not just at the domain level, like SimilarWeb's API).
I've migrated to Bing for all other web search needs (way cheaper and without volume limits like Google Custom Search), but Bing does not offer a "related:..." equivalent as far as I can tell.