I don't think there's anything further south than the Chong Mek / Vang Tao crossing, essentially between Ubon Ratchathani and Pakse. It's an easy crossing to make with direct buses going in both directions.
There's a great map here on Travelfish that shows all the border crossings for the entire region.
Just go into the "Visa and Border Crossings" forum and click on the first (sticky) post that says "please read this first". Scroll down a bit and there's the map, along with a ton of info on border crossings.