The Lonely Planet Guide to Laos, 5th Edition 2006, currently states that you can privately hire a Jumbo (taxi), to take you to see the Plain of Jars site 1 which is approximately 8 km from town. 

This is no longer the case.  As part of some scam between the local tour agents, hotels and the Government tour agency (rich getting richer springs to mind) you are now required to have a permit to visit these sites,  and surprise surprise, only a tour agent can get you this permit!

Now here’s the strange part.  You can still go to these sites privately either by cycle or motorbike but not by Jumbo.  It was explained to me; as a tour agent hires you a bicycle or motorbike they can also get you a permit.

Whats the permit for?  I have no idea, nobody asks you for it.  If the permit is required because there are still some UXO (unexploded ordinance) in the area, then its worthless as you are not escorted around the sites.

If you wish to visit the Plain of Jars sites 1, 2 and 3, it can be done quite easily, either by a tour group, or arranging a private tour through an agent.