BuiltWith - Find out the technology behind websites
Crunchbase - Crunchbase is a platform for discovering industry trends, investments, and news about companies around the world.
Wappalyzer - Wappalyzer is a technology profilers and leads data provider. Create lists of websites and contacts that use certain technologies.
Owler - Owler is a crowdsourced data model allowing users to follow, track, and research companies.
What CMS - WhatCMS.org looks at a variety of factors within a webpage to determine what CMS a website is using.
CB Insights - CB Insights is a company database for the venture capital and corporate development community.