Came across this web-site, having seen it in the newspaper, and thought people would be interested, if you get half as aggravated on the roads as I do.
It's called www.drivertalk.co.uk and well, this is what they say;
"It allows you to send personal messages to other drivers, simply by knowing their Vehicle Registration Number!
In short, the site uses Vehicle Registration Numbers instead of e-mail addresses. Register your vehicle today, and see who wants to talk to you!
Be it a road hog, or someone you fancy, now you can send them a personal message, and say what you wanna say.
Can the other driver reply? Yes. Whether your message is anonymous or not, drivers can reply using the Driver Talk messaging system."
(Copied from their FAQs, but it pretty much sums up the site - hope they don't mind!).