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.
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!
xZeitgeist - The most popular tweets categorized and ranked, every day
Do You Even List? - Auto updating Twitter list of people you interact with most
listshare - Create beautiful, interactive lists to share with the world.