The license plate wiki is interesting, but I fear the privacy implications. Sure, it may be legal, but it would still make me uneasy. I can see someone e.g. escaping their abusive spouse only to have their position given away online.
This would only work if people were also incentivized to post good stuff about you; for example on Ebay, even if you have a dick downvoting you unjustly, the amount of good feedback will probably make it up. But here, there's none of that.
This is a solid idea. An open version of Carfax/AutoCheck. And kind of like the way that Redfin keeps track of historical pricing information on real estate. Platewire is somewhat related (http://platewire.com/), but focuses on the driver instead of the vehicle.