BuiltWith - Find out the technology behind websites
Skuuudle - Discover, match and monitor your eCommerce competitors.
Wappalyzer - Wappalyzer is a technology profilers and leads data provider. Create lists of websites and contacts that use certain technologies.
Conversion Guard - Meet Conversion Guard, the all-one solution to safeguard your online business and protect the action of third-party plugins that promote other companies' products against your data & ad spending.
What CMS - WhatCMS.org looks at a variety of factors within a webpage to determine what CMS a website is using.
Similar Tech - SimilarTech offers a Sales Insights platform which helps companies uncover their ideal market by crawling the sourcecode of over 300M sites.