Carhaul Network

Carhaul Network is now online. Their subscription service gives you the opportunity to look for cars on their load board. They also have a detailed links page with DOT information relating to safety and compliance issues.

Other loadboards on the web:

Cars Arrive (I don't think they offer free trial)
I don't know these guys, but their website looks pretty slick.

CarloadUSA (does offer free trial)
This one is run by my buddy, Roland. Last time I checked, it had 2,300 loads to choose from.

Central Dispatch (does offer free trial)
I haven't looked at their board, but I've heard several guys say they use them.

If you're looking for extra cars to fill out a load, I think these types of services can be pretty useful. If you have three slots a month you fill because of them, that more than pays for the subscription.