Best site for free web conferences?

The board of the trade association I work with includes members from three states and DC. We’re all dedicated members of the group so we travel to all the events, too. With gas topping $4/gal in this area I’d really like to find a way to cut down on the travel for some of the members.

Any experiences good or bad? It would have to work for Windows and Mac

Ours is a not for profit group and our budget is tiny so free is best.

Thanks in advance.