Hunter TechLookup - Create lists of websites by technology for free
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!
listshare - Create beautiful, interactive lists to share with the world.
Top Lists - A leaderboard for Twitter lists
Siftery - Share products your company uses and get personalized recommendations
EmailMeTweets - Scheduled Twitter list notifications by email