Reactive Search - UI components for building Amazon / Yelp like Search
BuiltWith - Find out the technology behind websites
Searchkit - UI components for Elasticsearch
Wappalyzer - Wappalyzer is a technology profilers and leads data provider. Create lists of websites and contacts that use certain technologies.
Searchbase - Build Search UIs with drag and drop
What CMS - WhatCMS.org looks at a variety of factors within a webpage to determine what CMS a website is using.