BuiltWith - Find out the technology behind websites
Breakout List - A list of the fastest growing startups
Wappalyzer - Wappalyzer is a technology profilers and leads data provider. Create lists of websites and contacts that use certain technologies.
Mattermark - Ranking 800,000+ high growth Internet companies
What CMS - WhatCMS.org looks at a variety of factors within a webpage to determine what CMS a website is using.
Crunchbase - Crunchbase is a platform for discovering industry trends, investments, and news about companies around the world.