I spent a lot of time looking at free solutions for screen-sharing and ultimately chose TeamViewer because it has a lot to offer. The app is primarily for webinar-type meetings or remote controlling your machine but I've actually found it to be the best solution to simulcast/screenshare presentations given in one computer lab to the projector computers in all the others labs. This way if there is not enough room for students in one lab to attend a software workshop they can still participate in another space. This method works easily by starting a meeting and generating a code, which is then shared with the other computers to allow them to join the meeting. The UI is well designed and can be minimized to a tiny tab that latches on to the side of the screen, similar to GoToMeeting's control panel. This app is also a lifesaver when it comes to AV cabling - if it's too difficult to swap VGA for HDMI or whatever you need then the presenter can simply start a meeting with the projector computer. The ultimate reason why TeamViewer is superior to comparable apps is that its refresh rate and web speeds can handle orbiting a complex digital model - other apps got really choppy when trying to display a rotating Rhino model.