Google's online Hangouts, part of the companys Google Plus social network, also offers screen sharing for free. As in Skype, you'll only be able to see the remote party's screen, not control it, but you also get voice and video chat, too.
Go to Google Plus and click on the Hangouts icon (it looks like a circle with quotation marks in it, on the right side of the screen) if the sidebar isn't already visible. Click on “Start a Video Hangout” at the bottom of that column, and invite your contact via his or her Google account.
Once the other person has joined the hangout, he or she can start the screen sharing session by clicking on the Screenshare icon (a green screen with an arrow on it) in the left-hand column. As on Skype, you can select only a specific window, or choose to display your entire desktop. After selecting a window, have the person click Start Screenshare so the screen will be visible to everyone else in the hangout.