ICYMI, here is a VeloNews article that provides a great introduction to the Close Call Database. Read it here.
Welcome to the Close Call Database
A Community Project for Safer Cycling
Drivers that are hostile to cyclists are often serial offenders. Cataloging their aggression and documenting their hostility towards cyclists provides important information to fellow cyclists, local police and prosecutors. When you record an incident here, it will be shared with cyclists in your area. When other cyclists report an Incident, it will be shared with you. When problem drivers are identified this site will contact police where the Incidents have occurred so that they can intervene before another tragedy occurs. If that same driver does harm a cyclist at a later date, the information in the Incident Reports will increase the likelihood that justice is served.
If you would like to be notified of aggressive drivers in your area, or if you would like to report an incident with a hostile driver simply 
or register to create an account. [If you are already registered and are returning to review incident reports, access the login screen from the menu at the top of the page]
No one is going to solve this problem for us. We have to do it ourselves. Please choose to be part of the solution.
In case you need more, I wrote a post explaining my rationale and motivation. Still don't get it? Read this story at CNN.
Note, this site augments but does not replace current practices. If you have had a threatening encounter with a motorist you should still report it to the police!
Geographic Distribution of the Close Call Database Community
The database's effectiveness increases with membership and coverage. So if you are a cyclist and you have read this far and still haven't joined . . . then you are not helping! At all! Not even a little bit! This thing only works if you team up with the other cyclists in your area.
The map below shows the location of currently registered users. You can see that most users are concentrated in the United States and Great Britain but the service also has traction in other parts of the world. If there is nobody in your country or area yet don't be discouraged: every single country and location started off at zero. So just sign up, and encourage your cycling buddies to do the same. To get the most out of this service, you want to encourage other cyclists in your area to join. That's how you get 'eyes and ears' out on the road.
This map will allow you see your the current size of the network in your area. To do so, click on the round "cluster target" closest to the area that where you live. The reporting zone will typically encompass a 60 mile radius, so the numbers you see after two clicks into the map in your area is pretty representative of the number of other cyclists in your "network", but you can drill in deeper. If you drill in all the way, the final numbers are assigned to a city or zip code's geographic center.