I don't think GoToMeeting functions under Linux. Every time I've tried to connect, it keeps claiming that my system is unsupported and then stops. Webex does allow Linux clients, but with limited functionality. You can't share applications or your desktop, you can only view meetings. GoToMeeting is one of the reasons I have to keep a Windows machine around.
On the more obscure side, DimDim
does seem to support full Linux sharing and Yugma
claims that it is on their to-do list.