Hunter TechLookup - Create lists of websites by technology for free
Startup Toolkit - Interactive how-to guides to launch your startup
Techmap.io - Techmap is a workplace search engine to find companies using technologies (e.g., Rails, PHP, Java, etc.) that are used internally. Finally, a "Google for Workplaces" that helps people find a place to work with technologies they love!
Toolbird - Discover online tools that other startups use
WhatRuns - Extension that helps you identify technologies used on any website at the click of a button.
Techstars Entrepreneur's Toolkit - Online resource to help you learn about entrepreneurship