I've been creating a database that tracks these signals and corresponding tools. It would be great to hear and learn from others in this domain.
For example, Google alerts has tons of case studies.
BuiltWith can identify websites/companies that are using specific software/ technology and analyses customer churn between competitors.
Linkedin Premium can Track company headcount. Similarly, Mattermark can track, say, for example, Saas companies with headcount growth of 50% in the last 6 months.
What are the other tools with the most interesting applications in your opinion?