I have an Easy Rider. It might be better for an older dog, but it was not appropriate for my puppy because he moved around a lot and got tangled up in the car. I've only used it a couple of times and it cost me $40. Someone else mentioned that you can buy ones that attach to the car rather than the seatbelt, and I actually think this would be a lot better.
In my opinion, seatbelts aren't that great of an idea. A crate might be better.
I use the Champion Canine Seat Belt with Henry... he's had the same one since he outgrew his car crate and is now almost 4. VERY durable!! http://champk-9.com/CCSS.htm
Instead of the tether that comes with it, I use PetBuckle's Kwic Connect tether and keep it short so that he doesn't get tangled. What I like about this tether is that it connects to the bar you have in your car to anchor a child's carseat to, rather than to a seatbelt, so there's not as much play. I've seen them at our PetsMarts, or, you could order them online. http://www.immioutdoors.com/petbuckle/shop.htm
Casey has the JB Pet harness that Bearsdad posted the link to. We've had it for about 5 years now and it's worked well for her. She's always happy to put it on and go for a ride. ;D
The hardware on the Champion model looks like it might be stonger, though. Maybe I'll have to look into one of those.