Did I mention that it is free? iTalc allows you to manage your classroom from an instructor machine. You may want to lock all of the machines, show your screen to all of the students machine or project a students machine onto the overhead. You can do all this with iTalc. Now there developer has put some serious money into this, so lets support him. I will do my part through promotion at my work(s) and my blog.

Check it out:
http://italc.sourceforge.net