Daniel Woydig has created an web site called "Share Your Ride" that attempts to put riders and drivers together. Most of the focus is outside North America, but there are a few listings for this area. Daniel also has a variety of articles about hitchhiking and solo adventuring around the world.
Another ride-share site has launched this week -- this one was created by Jake Boshernitzan and is called Ridester. We don't know if either of this sites work -- but they are provided here as a public service.
Today, (01/10/07) received notice of yet another Rider Share Board -- this one is based in New York and managed by Aaron Craig. The owner shared some of the features of this service with us. GishiGo.com may be the best of the ones we have looked at. There is much more information posted about the route and the criteria of the offered ride or sought-after ride. Plus, both the rider and driver pay a small fee to use the service and the owner has built-in some good privacy protection safeguards.
There is another alternative -- which involves being the driver for one of the car transport services. One of the industry leaders in this regards is U-Ship and this page has a list of some of the types of driving gigs that can arise.
Again, we don't have any independent information about any of these services -- but would welcome your posts and recommendations if you have used any of them!