The only free bit of software I'm aware of is
. It's an open source virtual tabletop. I've never used it, and I don't know much about it.
There are two other options I'm aware of, but neither are free.
is a virtual tabletop program that is supposed to be fairly flexible. The software is not free, but I think it's a one time purchase. It is also available for a variety of operating systems.
Neverwinter Nights allows you to design and host a third edition DnD game. While you have to pay for the software, I don't think there is any additional cost. The DM hosts the game, and all the players connect over the internet. I have never used it, but I think I remember reading about people running games like that. It looks like a lot of work though. The DM has to create the maps, write scripts, and manage the whole thing.
I hope this helps.