Hunter TechLookup - Create lists of websites by technology for free
Listr - Listr lets you explore some of the best lists on Twitter, curated across your favorite categories.
Setalist - The easiest way to make awesome lists for the internet
Refined Twitter - A browser extension that improves the Twitter experience
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!
Vicariously.io - Create Twitter lists based on the follows of other users