Charles Wilson and I just used Fuze Box for about an hour this morning collaborating on a blog. It has a free for up to three users subscription. I logged in on a third laptop to see what the attendees (C.W.) see and the reaction time was incredibly fast, the screen resolution excellent.
I have only ever been on a GTM as a participant so I cant speak to the facilitator side of this. I did have the impression that the reaction time and crispness of the images/screen were better than GTM.
It's web based for the most part, the install is small. Don't know from Mac's, Mike... sorry.