You should take a look at Yugma (www.yugma.com). Yugma's free version allows you to share your desktop with up to 10 others while using the built-in instant message features.Great point Rebecca, thanks! There are a ton of free and open source alternatives to paid services and applications. In an earlier post - UPDATED: Educational Uses of Web 2.0 Based Applications - I pointed to Yugma and a number of synchronous learning tools.
Just added - Online Meeting-Screen/Application Sharing