some more examples that came to my mind (hope it’s not too late). On a general note, does it need to be fully open data, i.e. machine-readable, downloadable etc.? If not, it would make it easier to point you to some more examples
I assume you are aware of Safecast? They are pretty popular, and helped gathering information about nuclear radiation levels in the aftermath of Fukushima. Now they expanded in many countries and provide an open, crowdsourced database on several types of pollution.
I know that their data has been used both by activists as well as private companies and government (since they filled a monitoring gap at the time when Fukushima happened).
Local Authority Debt Audit (LADA)
Another interesting project is LADA (also featured in the Data and the City report. They are brokers between civic activists and financial auditors. Some of the group’s members are just about to finish a new website called Research for Action, and I really liked the guidance material they offer online (scroll to the bottom).
Civic databases like "The Counted"
You should check the report Changing What Counts again. It featured some projects where journalists retrieved information from newspaper articles on homicides and compiled those on a web interface. We also outlined in the report how people could use this information to spark debates about monitoring homicides more closely.
Hope this is useful