join.me is a site that allows multi-platform users (mac or pc) the ability to share screens via a free download. What's nice about it is that it is very easy to use and almost as useful as the built in screen sharing for a Mac.
I highly recommend checking out join.me for those looking to share their screen w/ others by clicking here. (This would be a nice thing to have when trying to help your friends/family troubleshoot their computer problems. Just download this app on both machines and then get to work.)
For a more detailed review check out Free Technology 4 Teachers.