SaaS Blocks by Apideck - +120 tools to build and augment your SaaS application
Wappalyzer - Wappalyzer is a technology profilers and leads data provider. Create lists of websites and contacts that use certain technologies.
Outseta - The "lean" tech stack for SaaS start-ups
BuiltWith - Find out the technology behind websites
Analytics Pack - A curated directory of the best startup metrics
What CMS - WhatCMS.org looks at a variety of factors within a webpage to determine what CMS a website is using.