A few groups have online incident report pages where you can enter information about a troublespot such as a close encounter with a motorist, and the group will log the involved parties (for possible future litigation or as evidence in a future criminal investigation) or even contact the motorist or forward the information to the police.
The Silicon Valley Bicycle Coalition, for example, inherited the PenBiPed's incident report tracker. This tracker has been used in the past to track behavior patterns of aggressive motorists. If multiple incidents are logged against a single vehicle, local law enforcement is contacted.
The Missouri Bicycle Federation has a motorist contact letter. Mo Bike Fed will look up and contact the motorist on behalf of cyclists.
In Colorado, you can dial "*CSP" (*277 for Colorado State Patrol) on your cell phone to report aggressive drivers. Motorists who are identified three times through the hotline receive a warning letter and any additional complaints result in a personal visit from a uniformed officer. You need a license plate number for this to work.
Joe Mizereck has recently created a worldwide incident tracker at Road Guardian. This is a tool cyclists can use to report, mark and share cycling incidents and trouble spots. This site’s value and benefit to the cycling world is dependent upon cyclists taking the time to report their cycling experiences: close calls, collisions, deaths, pot holes, harassment, doorings, etc. Joe is the guy who also created the yellow 3 Feet Please (Warning: Obnoxious audio starts automatically on site load) bicycle jersey with huge block letters asking for "3 FEET PLEASE IT'S THE LAW."