The premise behind license plate messaging is to allow drivers to send each other text messages without knowing the recipient's phone number. WhyHonk lets you call a number and leave a voice message with the license plate number and the message. It's then transcribed and sent, and the recipient can log onto the WhyHonk database to see the messages sent to them. SearchPlates requires you to sign up as a member (it's free), and license plate messages are sent using the online interface rather than a voice message.
I'm all about technological progression, but somehow the license plate messaging system promoted by WhyHonk and SearchPlates leaves something to be desired.